Binding domains directed against GPCR:G protein complexes and uses derived thereof

ABSTRACT

The present invention relates to the field of G protein coupled receptor (GPCR) structural biology and signaling. In particular, the present invention relates to binding domains directed against and/or specifically binding to GPCR:G protein complexes. Also provided are nucleic acid sequences encoding such binding domains and cells expressing or capable of expressing such binding domains. The binding domains of the present invention can be used as universal tools for the structural and functional characterization of G-protein coupled receptors in complex with downstream heterotrimeric G proteins and bound to various natural or synthetic ligands, for investigating the dynamic features of G protein activation, as well as for screening and drug discovery efforts that make use of GPCR:G protein complexes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. §371 ofInternational Patent Application PCT/EP2012/062036, filed Jun. 21, 2012,designating the United States of America and published in English asInternational Patent Publication WO 2012/175643 A2 on Dec. 27, 2012,which claims the benefit under Article 8 of the Patent CooperationTreaty and under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication Ser. No. 61/571,159, filed Jun. 21, 2011, and priority underArticle 8 of the PCT to European Patent Application Serial No.11181357.2, filed Sep. 15, 2011.

TECHNICAL FIELD

The present disclosure relates to the field of G protein coupledreceptor (GPCR) structural biology and signaling. In particular, thepresent disclosure relates to binding domains directed against and/orspecifically binding to GPCR:G protein complexes. Also provided arenucleic acid sequences encoding such binding domains and cellsexpressing or capable of expressing such binding domains. The bindingdomains of the present disclosure can be used as universal tools for thestructural and functional characterization of G-protein coupledreceptors in complex with downstream G proteins and bound to variousnatural or synthetic ligands, for investigating the dynamic features ofG protein activation, as well as for screening and drug discoveryefforts that make use of GPCR:G protein complexes.

STATEMENT ACCORDING TO 37 C.F.R. §1.821(c) OR (e)—SEQUENCE LISTINGSUBMITTED AS PDF FILE WITH A REQUEST TO TRANSFER CRF FROM PARENTAPPLICATION

Pursuant to 37 C.F.R. §1.821(c) or (e), a file containing a PDF versionof the Sequence Listing has been submitted concomitant with thisapplication, the contents of which are hereby incorporated by reference.The transmittal documents of this application include a Request toTransfer CRF from the parent application.

BACKGROUND

Seven-transmembrane receptors (7TMRs), also called G protein-coupledreceptors (GPCRs), are the largest class of receptors in the humangenome and are the most commonly targeted protein class for medicinaltherapeutics. Substantial progress has been made over the past threedecades in understanding diverse GPCRs, from pharmacology to functionalcharacterization in vivo. Recent high-resolution structural studies haveprovided insights into the molecular mechanisms of GPCR activation andconstitutive activity (e.g., Rasmussen et al., 2011). However, themolecular details of how GPCRs interact with and regulate the activityof their downstream targets are still lacking. The structures of GPCRsin complex with their downstream proteins are of great interest not onlybecause these interactions are pharmacologically relevant but alsobecause the atomic understanding of the intermolecular interactions arekey to unlocking the secrets of functional selectivity, the ability ofdifferent agonists to coerce distinct downstream effects from a singlekind of receptor. Recent structural data support the idea that GPCRs,despite their small size, are sophisticated allosteric machines withmultiple signaling outputs.

GPCRs, once activated, convey their signals in a GTP dependent mannervia a complex of three proteins known as heterotrimeric G proteins, orGαβγ. Binding of extracellular ligands to GPCRs modulates their capacityto catalyze GDP-GTP exchange in Gαβγ, thereby regulating theintracellular level of secondary messengers. The inactive Gαβγheterotrimer is composed of two principal elements, Gα-GDP and the Gβγheterodimer. Gβγ sequesters the switch II element on Gα such that it isunable to interact with second messenger systems, such as thoseinvolving cAMP, diacylglycerol and calcium. Activated GPCRs catalyze therelease of GDP from Gα, allowing GTP to bind and liberate the activatedGα-GTP subunit. In this state, switch II forms a helix stabilized by theγ-phosphate of GTP allowing it to interact with effectors such asadenylyl cyclase. Although much progress has been made in understandinghow Gα subunits interact with and regulate the activity of theirdownstream targets, it is not clear how activated GPCRs initiate thisprocess by catalyzing nucleotide exchange on Gαβγ.

Drug discovery efforts generally focus on small molecule ligands thatcompetitively bind to a particular catalytic or active site, usingstatic models of the target as a starting point. This method hasidentified and validated a multitude of viable active-site therapeuticsin use today. However, as reflected by the high failure rate of new drugcompounds (only an estimated 8% of phase I clinical therapeuticseventually gain Food and Drug Administration approval, at a conservativecost of $800 million per drug), many efforts are unsuccessful and oftentargets are abandoned once they are deemed undruggable (Lee & Craik,2009). A considerable part of these failures are due to the fact thatthe most prominent conformation of the target in question does notcorrespond to the druggable conformation to which a drug must bind to beeffective for the therapeutic indication. For example, efforts to obtainan agonist-bound active-state GPCR structure have proven difficult dueto the inherent instability of this state in the absence of a G protein.Recently it became possible to obtain structures of an active state of aGPCR, making use of conformationally selective stabilizing nanobodies orXAPERONES® that mimic G proteins and increase the affinity for agonistsat the orthosteric site (Rasmussen et al., 2011). This demonstrates thepower of XAPERONES® to lock the structure of the most challenging drugtargets in a therapeutically relevant conformation (Steyaert & Kobilka,2011) and their usefulness for directed drug discovery allowing tospecifically screen for potential drugs with higher sensitivity andselectivity towards a particular target (WO2012007593). One limitationof this technological approach is that for each GPCR target a specificstabilizing nanobody needs to be identified, which is not onlytime-consuming and costly, but also implies the availability ofdifferent tools, like biological material for immunization and selectionpurposes, amongst others.

DISCLOSURE

Unraveling the structural and functional features of GPCRs in complexwith their downstream heterotrimeric G proteins and bound to variousnatural and synthetic ligands is valuable both for understanding themechanisms of GPCR signal transduction as well as for drug discoveryefforts. For example, obtaining the structure of the active stateternary complex composed of the agonist, the GPCR and the G protein wasso far unsuccessful but highly desired in the structural biology ofsignal transduction because of its poorly understood biology.Crystallogenesis of this complex turned out to be extremely difficultbecause one partner (the receptor) needs detergents to be solubilizedwhile the G protein is unstable in detergents. Also, nucleotides thatare required for the formation of the complex also dissociate thecomplex in a transient process.

Therefore, and very unexpectedly, the inventors identified tools thatstabilize complexes of GPCRs and G proteins, which allowed them tocapture and purify such complexes, and finally to crystallize suchcomplexes. This will facilitate the identification of ligands or drugcompounds by, for example, structure based virtual screening or design,high-throughput screening or fragment-based drug discovery (see, e.g.,Example 10). More specifically, the inventors have identified bindingdomains, in particular immunoglobulin single variable domains, suitablefor the structural and functional analysis of an active state complexcomposed of an agonist, GPCR and G protein (see, e.g., Example 4-7).Interestingly, it was demonstrated that some of these GPCR:G proteincomplex-selective binding domains are specifically directed against theG protein, and not against the GPCR. For example, binding domains wereidentified that bind Gs at the interface of Gαs and Gβγ, without makingcontact with the beta-adrenergic receptor (see, e.g., Example 3), andwill thus be useful to capture and stabilize other Gs coupled receptors,as was demonstrated for the arginine vasopressin receptor 2 (V2R) (see,e.g., Example 9). Thus, it is a particular advantage that bindingdomains are directed against the G protein of a GPCR:G protein complex,since such a binding domain can be used as generic tool to stabilize andcapture active state complexes of the range of GPCRs that interact withthat particular G protein.

Thus, according to a first aspect, the disclosure relates to a bindingdomain that is directed against and/or specifically binds to a complexcomprising a GPCR and a G protein. More specifically, the binding domainas described herein binds with higher affinity to the GPCR:G proteincomplex as compared with binding to the G protein alone and/or to theGPCR alone, respectively. Also, the binding domain as described hereinenhances the affinity of a G protein for a GPCR. Thus, the presentdisclosure provides for binding domains directed against and/orspecifically binding to a conformational epitope of a complex comprisinga GPCR and a G protein, and stabilizes or locks the complex in aparticular conformational state, more specifically an activeconformational state.

In general, the binding domain as described herein may bind to anyconformational epitope that is made available or accessible by a complexof a GPCR and a G protein. These conformational epitopes may berepresented by the individual proteins comprised in the complex, and/ormay only be represented upon formation of the complex. Further, theseconformational epitopes may or may not be represented by the individualproteins alone. According to one particular embodiment, the bindingdomain as described herein specifically binds to the G protein comprisedin the complex, and not to the GPCR.

Typically, in nature, G proteins are in a nucleotide-bound form. Morespecifically, G proteins (or at least the α subunit) are bound to eitherGTP or GDP depending on the activation status of a particular GPCR, asdescribed further herein. Agonist binding to a GPCR promotesinteractions with the GDP-bound Gαβγ heterotrimer leading to exchange ofGDP for GTP on Gα, and the functional dissociation of the G protein intoGα-GTP and Gβγ subunits, which is needed for further intracellularsignaling. In a specific embodiment, the binding domain as describedherein specifically bind to and stabilize a GPCR:G protein complex inthe absence of nucleotides, more specifically the binding domains bindto and stabilize a GPCR:G protein complex wherein the G protein is in anucleotide free from. In a particular embodiment, the binding domain asdescribed herein will specifically bind to a conformational epitope atthe interface between the alpha and beta-gamma subunit of the G protein,and as such blocks the GDP/GTP binding site and interferes with GDP/GTPbinding. As such, and surprisingly, the binding domain as describedherein prevents or inhibits the dissociation of the GPCR:G proteincomplex in the presence of nucleotides, in particular guaninenucleotides or analogs thereof, such as GTPγS. Also, the binding domainas described herein prevents or inhibits binding of nucleotides to the Gprotein.

Preferably, the binding domain as described herein is directed againstand/or specifically binds to a complex comprising a GPCR, a G protein,and one or more receptor ligands. Typically, the receptor ligand will bean agonist or a positive allosteric modulator, or a combination thereof.

According to another preferred embodiment, the binding domain asdescribed herein is directed against and/or specifically binds to acomplex comprising a Gs protein coupled receptor and a Gs protein; or toa complex comprising a Gi protein coupled receptor and a Gi protein; orto complex comprising a Gt protein coupled receptor and a Gt protein; orto a complex comprising a Ggust protein coupled receptor and a Ggustprotein; or to a complex comprising a Gz protein coupled receptor and aGz protein; or to a complex comprising a Golf protein coupled receptorand a Golf protein; or to a complex comprising a Gq protein coupledreceptor and a Gq protein; or to a complex comprising a G12 coupledreceptor and a G12 protein; or to a complex comprising a G13 coupledreceptor and a G13 protein. The GPCR and/or the G protein as comprisedin the complex may be from the same or different species, in particularfrom a mammalian species. Preferably the GPCR is a human protein.

In general, a binding domain of the disclosure can be any non-naturallyoccurring molecule, or part thereof, that is able to specifically bindto a GPCR:G protein complex. Particularly, the binding domain asdescribed herein is an immunoglobulin single variable domain comprisingan amino acid sequence that comprises four framework regions (FR1 toFR4) and three complementarity-determining regions (CDR1 to CDR3),according to the following formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1).

Preferably, the binding domain as described herein is an immunoglobulinsingle variable domain derived from a Camelidae species, andparticularly is a nanobody or V_(H)H.

According to specific embodiments, the binding domain as describedherein is an immunoglobulin single variable domain comprising an aminoacid sequence that comprises four framework regions (FR1 to FR4) andthree complementarity-determining regions (CDR1 to CDR3), according tothe following formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1);and wherein CDR1 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 13-18,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 13-18,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 13-18,

and wherein CDR2 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 25-30,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 25-30,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 25-30,

and wherein CDR3 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 37-42,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 37-42,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 37-42.

In a particularly preferred embodiment, the present disclosure providesfor an immunoglobulin single variable domain comprising an amino acidsequence that comprises four framework regions (FR1 to FR4) and threecomplementarity-determining regions (CDR1 to CDR3), according to thefollowing formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1);

-   -   wherein CDR1 is SEQ ID NO: 13;    -   wherein CDR2 is SEQ ID NO: 25; and    -   wherein CDR3 is SEQ ID NO: 37.

According to a very specific embodiment, the binding domain, inparticular the immunoglobulin single variable domain, has an amino acidsequence selected from the group consisting of consisting of SEQ ID NOs:1 to 6.

Further the binding domains as described herein may also be comprised ina polypeptide. Also, the binding domains may be immobilized on a solidsupport.

In one particular aspect, the present disclosure relates to a bindingdomain that is directed against and/or specifically binds to a Gprotein.

Another aspect of the disclosure envisages a complex comprising abinding domain as described herein. In particular, the complex comprisesa GPCR, a G protein, and optionally a receptor ligand. In certainapplications, the complex as described herein is crystalline.

Further, the disclosure also provides for nucleic acid sequences, inparticular a nucleic acid sequence encoding any amino acid sequence ofany of the binding domains as described herein, as well as recombinantvectors comprising any of the nucleic acid sequences as describedherein. Particularly preferred aspects of the disclosure are cellscomprising any of the vectors or nucleic acids as described herein, andas such, that can express or are capable of expressing a GPCR and/or a Gprotein. Cell cultures of cells according to the disclosure as well asmembrane preparations derived thereof are also within the scope of thepresent invention.

The herein described binding domains, complexes and cells may be usefulin a variety of contexts and applications. Thus, accordingly, one aspectof the disclosure relates to the use of a binding domain as describedherein to stabilize a complex comprising a GPCR and a G protein, andoptionally a receptor ligand, in a functional conformational state, morespecifically in an active conformational state. In one specificembodiment, the binding domains as described herein can be used toprevent dissociation of the complex in the presence of nucleotides, inparticular guanine nucleotides or analogs thereof, such as GTPγS.Binding domains as tools to stabilize GPCR:G protein complexes and blockthe GPCR in a functional conformational state, preferably an activeconformational state, are thus very useful for a range of applications,as outlined hereafter.

Disclosed is the use of a binding domain as described herein tocrystallize and/or to solve the structure of a complex comprising a GPCRand a G protein, and optionally a receptor ligand.

Also envisaged within the scope of the present disclosure is to use abinding domain as described herein or a cell or a membrane preparationderived thereof, as described herein, to screen for compounds thatmodulate the signaling activity of the GPCR.

Further, the binding domains as described herein may be used to captureone or more interacting proteins, in particular proteins that interactwith the G protein and/or with the GPCR.

According to specific embodiments, the present disclosure provides for amethod of capturing and/or purifying a complex comprising a GPCR and a Gprotein, the method comprising the steps of:

-   -   a) Providing a binding domain as described herein, and    -   b) Allowing the binding domain to bind to a complex comprising a        GPCR and a G protein and optionally a receptor ligand, and    -   c) Optionally, isolating the complex formed in step b).

In another specific embodiment, the present disclosure relates to amethod of determining the crystal structure of a complex comprising aGPCR and a G protein, the method comprising the steps of:

-   -   a) Providing a binding domain as described herein, and    -   b) Allowing the binding domain to bind to a complex comprising a        GPCR and a G protein and optionally a receptor ligand, and    -   c) Crystallizing the complex formed in step b).

Some of the binding domains as described herein may have therapeuticutility. Thus, it is also an object of the disclosure to use the bindingdomains as described herein to modulate GPCR receptor signaling, inparticular G protein-mediated GPCR receptor signaling.

The present disclosure further encompasses a method of producing abinding domain directed against and/or specifically binding to a complexcomprising a GPCR and a G protein, the method comprising the steps of:

-   -   a) Expressing in a suitable cellular expression system a nucleic        acid as described herein, and optionally,    -   b) Isolating and/or purifying the binding domain.

Another aspect of the present disclosure relates to a method ofscreening for binding domains directed against and/or specificallybinding to a complex comprising a GPCR and a G protein, the methodcomprising the steps of:

-   -   a) Providing a plurality of binding domains, and    -   b) Screening the plurality of binding domains for a binding        domain that binds to a complex comprising a GPCR and a G        protein, and    -   c) Isolating the binding domain that binds to the complex.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1: G protein cycle for the β₂AR:Gs complex. Panel a, Extracellularagonist binding to the β₂AR leads to conformational rearrangements ofthe cytoplasmic ends of transmembrane segments that enable the Gsheterotrimer (α, β, and γ) to bind the receptor (R, R*). GDP is releasedfrom the α subunit upon formation of R:G complex. The GTP binds to thenucleotide-free α subunit resulting in dissociation of the α and βγsubunits from the receptor. The subunits regulate their respectiveeffector proteins adenylyl cyclase (AC) and Ca²⁺ channels. The Gsheterotrimer reassembles from α and βγ subunits following hydrolysis ofGTP to GDP in the α subunit. Panel b, The purified nucleotide-freeβ₂AR:Gs protein complex maintained in detergent micelles. The Gsαsubunit consists of two domains, the Ras domain (αRas) and the α-helicaldomain (αAH). Both are involved in nucleotide binding. In thenucleotide-free state, the αAH domain has a variable position relativethe αRas domain.

FIG. 2: Formation of a stable β₂AR:Gs complex. A stable β₂AR:Gs complexwas achieved by the combined effects of: 1) binding a high affinityagonist to the receptor with an extremely slow dissociation rate (asdescribed in Rasmussen et al., 2011); 2) formation of a nucleotide freecomplex in the presence of apyrase that hydrolyses released GDPpreventing it from rebinding and causing a less stable R:G interaction;and 3) detergent exchange of DDM for MNG-3 that stabilizes the complex.

FIGS. 3A-3G: Effect of nucleotide analogs, pH, and nanobodies on thestability of the β₂AR:Gs complex. FIG. 3A, Analytical gel filtrationshowing that nucleotides GDP and GTPγS (0.1 mM) causes dissociation ofthe β₂AR-365:Gs complex. FIG. 3B, The phosphates pyrophosphate andforcarnet (used at 5 mM) resembling the nucleotide phosphate groups didnot cause disruption of the complex. They served as additives as theyimproved crystal growth of both the T4L-β2AR:Gs complex (withoutnanobodies), T4L-β2AR:Gs:Nb37, and T4L-β2AR:Gs:Nb35. FIG. 3C, The pHlimit was determined to guide the preparation of crystallizationscreens. For the same purpose the effect of ionic strength (data notshown) was determined using NaCl at various concentrations. The complexis stable in 20, 100, and 500 mM but dissociates at 2.5 M NaCl. FIG. 3D,Nanobody 35 (Nb35, red broken line) binds to the T4L-β2AR:Gs:BI167107ternary complex (blue solid line) to form the R:G:Nb35 complex (redsolid line) which is insensitive to GTPγS treatment (green solid line)in contrast to the treated R:G complex (green broken line). Nb35 andNb37 bind separate epitopes on the Gs heterotrimer to form aR:G:Nb35:Nb37 complex (purple solid line). FIG. 3E, Nanobody 36 (Nb36,red broken line) binds to the to the R:G complex (black solid line) toform the R:G:Nb36 complex (red solid line) which is less sensitive toGTPγS treatment (green solid line). Nb36 and Nb37 bind separate epitopeson the Gs heterotrimer to form a R:G:Nb36:Nb37 complex (purple solidline). FIG. 3F, Nanobody 37 (Nb37, green line) binds to the to the R:Gcomplex (black solid line) to form the R:G:Nb37 complex (red solidline). FIG. 3G, The R:G:Nb37 complex is insensitive to GTPγS treatment(blue solid line) in contrast to the treated R:G complex (blue brokenline).

FIG. 4: Stabilizing effect of MNG-3 on the R:G complex. Panel a)Analytical gel filtration of β₂AR-365:Gs complexes purified in DDM (inblack), MNG-3 (in blue), or two MNG-3 analogs (in red and green)following incubation for 48 hrs at 4° C. In contrast to DDM the R:Gcomplexes are stable in the MNG detergents. Panel b) Effect of dilutingunliganded purified β2AR in DDM or MNG-3 below the critical micelleconcentration (CMC) of the detergent as assayed by 3H-dihydro alprenolol(3H-DHA) saturation binding. Diluting β2AR maintained in DDM 1000-foldbelow the CMC cause loss in 3H-DHA binding (black data points) after 20sec. In contrast β2AR in MNG-3 diluted 1000-fold below the CMCmaintained full ability to bind the radioligand after 24 hrs.

FIG. 5: Purity and homogeneity of the R:G complex. Panel a) AnalyticalSDS-PAGE/Coomassie blue stain of samples obtained at various stages ofT4L-β₂AR:Gs purification. BI167107 agonist bound, dephosphorylated, anddeglycosylated receptor in excess amount of Gs heterotrimer is used foroptimal coupling efficiency with the functional fraction of the Gprotein. Functional purification of Gs is archived through itsinteraction with the immobilized receptor on the M1 resin whilenon-functional/non-binding Gs washes out. Panel b) A representativeelution profile of one of four consecutive preparative gel filtrationswith fractionation indicated in red. Fractions containing the R:Gcomplex were pooled within the indicated dashed lines, spin concentratedand analyzed for purity and homogeneity by SDS-PAGE/Coomassie blue (a,second last lane to the right), gel filtration in Panel c), and by anionexchange chromatography in Panel d). Upper panel shows elution profilefrom analytical IEC of β2AR-365:Gs complex that was treated with λPPaseprior to complex formation in comparison with complex which was notdephosphorylated resulting in a heterogeneous preparation (lower panel).The less homogeneous material in the fractions outside the indicateddashed lines in Panel b) were used for analytical gel filtrationexperiments of the R:G complex in presence of various chemicals(examples in FIG. 1).

FIG. 6: Flow-chart of the purification procedures for preparing R:Gcomplex with Nb35.

FIG. 7: Purification of Nb35 and determination of R:G:Nb mixing ratio.Panel a) Preparative ion exchange chromatography following nickelaffinity chromatography purification of Nanobody 35 (Nb35). The nanobodyeluted in two populations (shown in red) as a minor peak and a majorhomogeneous peak which was collected, spin concentrated, and used forcrystallography following determination of proper mixing ratio with theR:G complex as shown in Panel b). Panel b) The agonist bound T4L-β2AR:Gscomplex was mixed with slight excess of Nb35 (1 to 1.2 molar ratio ofR:G complex to Nb35) on the basis of their protein concentrations andverified by analytical gel filtration.

FIG. 8: Crystals of the T4L-β2AR:Gs:Nb35 complex in sponge-likemesophase.

FIG. 9: Overall structure of the β₂AR:Gs complex. Panel a, Latticepacking of the complex shows alternating layers of receptor and Gprotein within the crystal. Abundant contacts are formed among proteinswithin the aqueous layers. Panel b, The overall structure of theasymmetric unit contents shows the β₂AR (green) bound to an agonist(yellow spheres) and engaged in extensive interactions with Gsα(orange). Gαs together with Gβ (cyan) and Gγ (purple) constitute theheterotrimeric G protein Gs. A Gs binding nanobody (Nb35, red) binds theG protein between the α and β subunits. The nanobody (Nb35) facilitatescrystallization, as does T4 lysozyme (magenta) fused to the aminoterminus of the β₂AR. Panel c, The biological complex omittingcrystallization aids, showing its location and orientation within a cellmembrane.

FIGS. 10A-10C: Interactions of Nb35 with Gs within the BI-167107 boundT4L-β2AR:Gs:Nb35 complex. FIG. 10A, Two representative views on theinteractions of CDR1 (space filling representation) of Nb35 (red) withGP (space filling, cyan). FIG. 10B, Two representative views on theinteractions of CDR3 (space filling representation) of Nb35 (red) withGαS (space filling, orange) and Gβ (space filling, cyan). By interactingwith GαS and GP, Nb35 may reduce the conformational flexibility of thecomplex. FIG. 10C, Two representative views on the interactions of theframework regions of Nb35 (space filling representation, red) with GαS(orange).

FIGS. 11A and 11B: Crystal contacts between Nb35 and GαS subunits ofadjacent complexes. Crystal contacts involving Nb35 (red, space-fillingrepresentation) and GαS (orange) of the −x,y−1/2,−z+1 symmetry relatedcomplex (FIG. 11A) and the x,y−1,z symmetry related complex (FIG. 11B).

FIG. 12: Comparison of active and inactive β2AR structures. Panel a,Side and cytoplasmic views of the β2AR:Gs structure (green) compared tothe inactive carazolol-bound β2AR structure (blue; Rosenbaum et al.,2007). Significant structural changes are seen for the intracellulardomain of TM5 and TM6. TM5 is extended by two helical turns while TM6 ismoved outward by 14 Å as measured at the α-carbons of Glu268 (yellowarrow) in the two structures. Panel b, β2AR:Gs compared with thenanobody-stabilized active state β2AR:Nb80 structure (orange, Rasmussenet al., 2011). Panel c, The positions of residues in the E/DRY and NPxxYmotifs and other key residues of the β2AR:Gs and β2AR:Nb80 structures asseen from the cytoplasmic side. All residues occupy very similarpositions except Arg131 which in the β2AR:Nb80 structure interacts withthe nanobody.

FIG. 13: Views of electron density for residues in the R:G interface.Panel a) The D/ERY motif at the cytoplasmic end of TM3. Panel b) Packinginteraction between Arg131 of the E/DRY motif and Tyr391 of C-terminalGαS. Panel c) The NPxxY in the cytoplasmic end of TM7. Panel d)Interactions of Thr68 and Tyr141 with Asp130 of the E/DRY motif. Phe139of IL2 is buried in a hydrophobic pocket in GαS. Panel e) The β1-α1 loop(P-loop) of GαS involved in nucleotide binding. Electron density mapsare 2Fo-Fc maps contoured at 1 sigma.

FIG. 14: Receptor:G protein interactions. Panels a and b: The α5-helixof GαS docks into a cavity formed on the intracellular side of thereceptor by the opening of transmembrane helices 5 and 6. Panel a:Within the transmembrane core, the interactions are primarily non-polar.An exception involves packing of Tyr391 of the α5-helix against Arg131of the conserved DRY sequence in TM3 (see also FIG. 13). Arg131 alsopacks against Tyr of the conserved NPxxY sequence in TM7. Panel b: Asα5-helix exits the receptor it forms a network of polar interactionswith TM5 and TM3. Panel c: Receptor residues Thr68 and Asp130 interactwith the IL2 helix of the β₂AR via Tyr141, positioning the helix so thatPhe139 of the receptor docks into a hydrophobic pocket on the G proteinsurface, thereby structurally linking receptor-G protein interactionswith the highly conserved DRY motif of the β₂AR.

FIG. 15: Conformational changes in Gαs. Panel a, A comparison of Gαs inthe β₂AR:Gs complex (orange) with the GTPγS-bound Gαs (grey) (PDB ID: 1AZT; Sunahara et al., 1997). GTPγS is shown as spheres. The helicaldomain of Gαs (GαsAH) exhibits a dramatic displacement relative to itsposition in the GTPγS-bound state. Panel b, The α5-helix of Gαs isrotated and displaced toward the β₂AR, perturbing the β6-α5 loop whichotherwise forms part of the GTPγS binding pocket. Panel c, The β1-α1loop (P-loop) and β6-α5 loop of Gαs interact with the phosphates andpurine ring, respectively, of GTPγS in the GTPγS-Gαs structure. Panel d,The β1-α1 and β6-α5 loops are rearranged in the nucleotide-free β₂AR:Gsstructure.

FIG. 16: Nb37 inhibits GTPγS binding to Gαs. Bodipy-GTPγS (100 nM) wasincubated with 1 μM purified Gαs and the fluorescence increase measuredin real time in the presence of increasing concentrations of Nb37.

FIG. 17: Nb35 does not affect GTPγS binding to Gαs. Bodipy-GTPγS (100nM) was incubated with 1 μM purified Gαs and the fluorescence increasemeasured in real time in the presence of increasing concentrations ofNb35.

FIG. 18: Nb35 inhibits GTPγS binding to the Gsαβγ heterotrimer.Bodipy-GTPγS (100 nM) was incubated with 1 μM purified Gsαβγheterotrimer and the fluorescence increase measured in real time in thepresence of increasing concentrations of Nb35.

FIG. 19: Purification of a stable AVP:NT4LV2R:Gs complex. Panel A,schematic representation of the purification of the AVP:NT4LV2R:Gscomplex using Ni-NTA followed by FLAG-tag affinity purification. Panelb, SEC chromatogram of the affinity purified AVP:NT4LV2R:Gs. Panel c,SDS-page to monitor the purification scheme. lane 1: flow trough of theFLAG-tag affinity column; lane 2: mix of AVP, NT4LV2R and Gs, prior topurification; lane 3: molecular marker; lane 4: AVP:NT4LV2R:Gs complexeluted from SEC; lane 5: AVP:NT4LV2R:Gs complex after Ni-NTA followed byFLAG-tag affinity purification.

FIG. 20: Stability of the AVP:NT4LV2R:Gs complex monitored by SEC.Dashed line: SEC chromatogram of the AVP:NT4LV2R:Gs complex after 24hours incubation on ice. Blue line: SEC chromatogram of theAVP:NT4LV2R:Gs complex after 48 hours incubation on ice. Red line, SECchromatogram of AVP:NT4LV2R:Gs after incubation of the complex with 10μM of the antagonist SR121463.

DETAILED DESCRIPTION

Definitions

The present disclosure will be described with respect to particularembodiments and with reference to certain drawings but the disclosure isnot limited thereto but only by the claims. Any reference signs in theclaims shall not be construed as limiting the scope. The drawingsdescribed are only schematic and are non-limiting. In the drawings, thesize of some of the elements may be exaggerated and not drawn on scalefor illustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Where an indefinite or definite article is used when referring toa singular noun, e.g., “a” “an,” or “the,” this includes a plural ofthat noun unless something else is specifically stated. Furthermore, theterms first, second, third and the like in the description and in theclaims, are used for distinguishing between similar elements and notnecessarily for describing a sequential or chronological order. It is tobe understood that the terms so used are interchangeable underappropriate circumstances and that the embodiments of the disclosuredescribed herein are capable of operation in other sequences thandescribed or illustrated herein.

Unless otherwise defined herein, scientific and technical terms andphrases used in connection with the present disclosure shall have themeanings that are commonly understood by those of ordinary skill in theart. Generally, nomenclatures used in connection with, and techniques ofmolecular and cellular biology, genetics and protein and nucleic acidchemistry and hybridization described herein are those well known andcommonly used in the art. The methods and techniques of the presentdisclosure are generally performed according to conventional methodswell known in the art and as described in various general and morespecific references that are cited and discussed throughout the presentspecification unless otherwise indicated. See, for example, Sambrook etal., Molecular Cloning: A Laboratory Manual, 2d ed., Cold Spring HarborLaboratory Press, Cold Spring Harbor, N.Y. (1989); Ausubel et al.,Current Protocols in Molecular Biology, Greene Publishing Associates(1992, and Supplements to 2002).

The term “binding domain” or “protein binding domain” refers generallyto any non-naturally occurring molecule, or part thereof, that is ableto bind a protein or peptide using specific intermolecular interactions.A variety of molecules can function as protein binding domains,including, but not limited to, proteinaceous molecules (protein,peptide, protein-like or protein containing), nucleic acid molecules(nucleic acid, nucleic acid-like, nucleic acid containing), andcarbohydrate molecules (carbohydrate, carbohydrate-like, carbohydratecontaining). A more detailed description can be found further in thespecification.

The terms “polypeptide,” “protein,” or “peptide” are usedinterchangeably herein, and refer to a polymeric form of amino acids ofany length, which can include coded and non-coded amino acids,chemically or biochemically modified or derivatized amino acids, andpolypeptides having modified peptide backbones.

As used herein, the term “protein complex” or simply “complex,” refersto a group of two or more associated polypeptide chains. Proteins in aprotein complex are linked by non-covalent protein-protein interactions.The “quaternary structure” is the structural arrangement of theassociated folded proteins in the protein complex. It will be understoodthat the complex may be a multimeric complex comprising two, three,four, five, six or more polypeptides. Also, the complex may additionallycomprise a non-proteinaceous molecule.

As used herein, the terms “nucleic acid molecule,” “polynucleotide,”“polynucleic acid,” or “nucleic acid” are used interchangeably and referto a polymeric form of nucleotides of any length, eitherdeoxyribonucleotides or ribonucleotides, or analogs thereof.Polynucleotides may have any three-dimensional structure, and mayperform any function, known or unknown. Non-limiting examples ofpolynucleotides include a gene, a gene fragment, exons, introns,messenger RNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA,recombinant polynucleotides, branched polynucleotides, plasmids,vectors, isolated DNA of any sequence, control regions, isolated RNA ofany sequence, nucleic acid probes, and primers. The nucleic acidmolecule may be linear or circular.

As used herein, the term “ligand” or “receptor ligand” means a moleculethat specifically binds to a GPCR, either intracellularly orextracellularly. A ligand may be, without the purpose of beinglimitative, a protein, a (poly)peptide, a lipid, a small molecule, aprotein scaffold, an antibody, an antibody fragment, a nucleic acid, acarbohydrate. A ligand may be synthetic or naturally occurring. The term“ligand” includes a “native ligand” which is a ligand that is anendogenous, natural ligand for a native GPCR. In most cases, a ligand isa “modulator” that increases or decreases an intracellular response whenit is in contact with, for example binds to, a GPCR that is expressed ina cell. Examples of ligands that are modulators include agonists,partial agonists, inverse agonists, and antagonists, of which a moredetailed description can be found further in the specification.

The term “conformation” or “conformational state” of a protein refersgenerally to the range of structures that a protein may adopt at anyinstant in time. One of skill in the art will recognize thatdeterminants of conformation or conformational state include a protein'sprimary structure as reflected in a protein's amino acid sequence(including modified amino acids) and the environment surrounding theprotein. The conformation or conformational state of a protein alsorelates to structural features such as protein secondary structures(e.g., α-helix, β-sheet, among others), tertiary structure (e.g., thethree dimensional folding of a polypeptide chain), and quaternarystructure (e.g., interactions of a polypeptide chain with other proteinsubunits). Post-translational and other modifications to a polypeptidechain such as ligand binding, phosphorylation, sulfation, glycosylation,or attachments of hydrophobic groups, among others, can influence theconformation of a protein. Furthermore, environmental factors, such aspH, salt concentration, ionic strength, and osmolality of thesurrounding solution, and interaction with other proteins andco-factors, among others, can affect protein conformation. Theconformational state of a protein may be determined by either functionalassay for activity or binding to another molecule or by means ofphysical methods such as X-ray crystallography, NMR, or spin labeling,among other methods. For a general discussion of protein conformationand conformational states, one is referred to Cantor and Schimmel,Biophysical Chemistry, Part I: The Conformation of BiologicalMacromolecules, W.H. Freeman and Company, 1980, and Creighton, Proteins:Structures and Molecular Properties, W.H. Freeman and Company, 1993. A“specific conformation” or “specific conformational state” is any subsetof the range of conformations or conformational states that a proteinmay adopt.

As used herein, a “functional conformation” or a “functionalconformational state,” refers to the fact that proteins possessdifferent conformational states having a dynamic range of activity, inparticular ranging from no activity to maximal activity. Examples offunctional conformational states include active conformations andinactive conformations. It should be clear that “a functionalconformational state” is meant to cover any conformational state of aGPCR, having any activity, including no activity; and is not meant tocover the denatured states of proteins. A particular class of functionalconformations that is envisaged here is a “druggable conformation” andgenerally refers to a unique therapeutically relevant conformationalstate of a target protein. As an illustration, the active conformationof the β2 adrenergic receptor corresponds to the druggable conformationof this receptor for the treatment of asthma. It will thus be understoodthat drugability is confined to particular conformations depending onthe therapeutic indication.

The wording “locking” or “trapping” or “fixing” or “freezing” withrespect to a functional conformational state of a GPCR (as definedherein), as used herein, refers to the retaining or holding of a GPCR ina subset of the possible conformations that it could otherwise assume,due to the effects of the interaction of the GPCR:G protein complex withthe binding domain according to the invention. Accordingly, a proteinthat is “conformationally trapped” or “conformationally fixed” or“conformationally locked” or “conformationally frozen,” as used herein,is one that is held in a subset of the possible conformations that itcould otherwise assume, due to the effects of the interaction of theGPCR:G protein complex with the binding domain according to theinvention. Within this context, a binding domain that specifically orselectively binds to a specific conformation or conformational state ofa protein refers to a binding domain that binds with a higher affinityto a protein in a subset of conformations or conformational states thanto other conformations or conformational states that the protein mayassume. One of skill in the art will recognize that binding domains thatspecifically or selectively bind to a specific conformation orconformational state of a protein will stabilize this specificconformation or conformational state.

As used herein, the terms “complementarity-determining region” or “CDR”within the context of antibodies refer to variable regions of either H(heavy) or L (light) chains (also abbreviated as V_(H) and V_(L),respectively) and contains the amino acid sequences capable ofspecifically binding to antigenic targets. These CDR regions account forthe basic specificity of the antibody for a particular antigenicdeterminant structure. Such regions are also referred to as“hypervariable regions.” The CDRs represent non-contiguous stretches ofamino acids within the variable regions but, regardless of species, thepositional locations of these critical amino acid sequences within thevariable heavy and light chain regions have been found to have similarlocations within the amino acid sequences of the variable chains. Thevariable heavy and light chains of all canonical antibodies each havethree CDR regions, each non-contiguous with the others (termed L1, L2,L3, H1, H2, H3) for the respective light (L) and heavy (H) chains. Inparticular, immunoglobulin single variable domains, such as nanobodies(as defined further herein), generally comprise a single amino acidchain that comprises four “framework sequences or regions” or FRs(termed FR1, FR2, FR3, FR4) and three complementarity-determiningregions” or CDRs (termed CDR1, CDR2, CDR3), each non-contiguous with theothers. The delineation of the CDR sequences (and thus also of the FRsequences) is based on the IMGT unique numbering system for V-domainsand V-like domains (Lefranc et al., 2003).

An “epitope,” as used herein, refers to an antigenic determinant of apolypeptide. An epitope could comprise three amino acids in a spatialconformation, which is unique to the epitope. Generally an epitopeconsists of at least 4, 5, 6, 7 such amino acids, and more usually,consists of at least 8, 9, 10 such amino acids. Methods of determiningthe spatial conformation of amino acids are known in the art, andinclude, for example, x-ray crystallography and multi-dimensionalnuclear magnetic resonance.

A “conformational epitope,” as used herein, refers to an epitopecomprising amino acids in a spatial conformation that is unique to afolded 3-dimensional conformation of the polypeptide. Generally, aconformational epitope consists of amino acids that are discontinuous inthe linear sequence that come together in the folded structure of theprotein. However, a conformational epitope may also consist of a linearsequence of amino acids that adopts a conformation that is unique to afolded 3-dimensional conformation of the polypeptide (and not present ina denatured state). In protein complexes, conformational epitopesconsist of amino acids that are discontinuous in the linear sequences ofone or more polypeptides that come together upon folding of thedifferent folded polypeptides and their association in a uniquequaternary structure. Similarly, conformational epitopes may here alsoconsist of a linear sequence of amino acids of one or more polypeptidesthat come together and adopt a conformation that is unique to thequaternary structure.

The term “specificity,” as used herein, refers to the ability of abinding domain, in particular an immunoglobulin, such as an antibody, oran immunoglobulin fragment, such as a nanobody, to bind preferentiallyto one antigen, versus a different antigen, and does not necessarilyimply high affinity (as defined further herein). A binding domain, inparticular an immunoglobulin, such as an antibody, or an immunoglobulinfragment, such as a nanobody, that can specifically bind to and/or thathas affinity for a specific antigen or antigenic determinant (e.g.,epitope) is said to be “against” or “directed against” the antigen orantigenic determinant. A binding domain according to the disclosure issaid to be “cross-reactive” for two different antigens or antigenicdeterminants if it is specific for both these different antigens orantigenic determinants.

The term “affinity,” as used herein, refers to the degree to which abinding domain, in particular an immunoglobulin, such as an antibody, oran immunoglobulin fragment, such as a nanobody, binds to an antigen soas to shift the equilibrium of antigen and binding domain toward thepresence of a complex formed by their binding. Thus, for example, wherean antigen and antibody (fragment) are combined in relatively equalconcentration, an antibody (fragment) of high affinity will bind to theavailable antigen so as to shift the equilibrium toward highconcentration of the resulting complex. The dissociation constant(K_(d)) is commonly used to describe the affinity between the proteinbinding domain and the antigenic target. Typically, the dissociationconstant is lower than 10⁻⁵ M. Preferably, the dissociation constant islower than 10⁻⁶ M, more preferably, lower than 10⁻⁷ M. Most preferably,the dissociation constant is lower than 10⁻⁸M.

The terms “specifically bind” and “specific binding,” as used herein,generally refers to the ability of a binding domain, in particular animmunoglobulin, such as an antibody, or an immunoglobulin fragment, suchas a nanobody, to preferentially bind to a particular antigen that ispresent in a homogeneous mixture of different antigens. In certainembodiments, a specific binding interaction will discriminate betweendesirable and undesirable antigens in a sample, in some embodiments morethan about 10 to 100-fold or more (e.g., more than about 1000- or10,000-fold).

The terms “specifically bind,” “selectively bind,” “preferentiallybind,” and grammatical equivalents thereof, are used interchangeablyherein. The terms “conformational specific” or “conformationalselective” are also used interchangeably herein.

A “deletion” is defined here as a change in either amino acid ornucleotide sequence in which one or more amino acid or nucleotideresidues, respectively, are absent as compared to an amino acid sequenceor nucleotide sequence of a parental polypeptide or nucleic acid. Withinthe context of a protein or a fragment thereof, a deletion can involvedeletion of about 2, about 5, about 10, up to about 20, up to about 30or up to about 50 or more amino acids. A protein or a fragment thereofmay contain more than one deletion. Within the context of a GPCR, adeletion may in particular be a loop deletion, or an N- and/orC-terminal deletion.

An “insertion” or “addition” is that change in an amino acid ornucleotide sequence which has resulted in the addition of one or moreamino acid or nucleotide residues, respectively, as compared to an aminoacid sequence or nucleotide sequence of a parental protein. “Insertion”generally refers to addition to one or more amino acid residues withinan amino acid sequence of a polypeptide, while “addition” can be aninsertion or refer to amino acid residues added at an N- or C-terminus,or both termini. Within the context of a protein or a fragment thereof,an insertion or addition is usually of about 1, about 3, about 5, about10, up to about 20, up to about 30 or up to about 50 or more aminoacids. A protein or fragment thereof may contain more than oneinsertion.

A “substitution,” as used herein, results from the replacement of one ormore amino acids or nucleotides by different amino acids or nucleotides,respectively as compared to an amino acid sequence or nucleotidesequence of a parental protein or a fragment thereof. It is understoodthat a protein or a fragment thereof may have conservative amino acidsubstitutions which have substantially no effect on the protein'sactivity. By conservative substitutions is intended combinations such asgly, ala; val, ile, leu, met; asp, glu; asn, gln; ser, thr; lys, arg;cys, met; and phe, tyr, trp.

The term “sequence identity” as used herein refers to the extent thatsequences are identical on a nucleotide-by-nucleotide basis or an aminoacid-by-amino acid basis over a window of comparison. Thus, a“percentage of sequence identity” is calculated by comparing twooptimally aligned sequences over the window of comparison, determiningthe number of positions at which the identical nucleic acid base (e.g.,A, T, C, G, I) or the identical amino acid residue (e.g., Ala, Pro, Ser,Thr, Gly, Val, Leu, Ile, Phe, Tyr, Trp, Lys, Arg, His, Asp, Glu, Asn,Gln, Cys and Met) occurs in both sequences to yield the number ofmatched positions, dividing the number of matched positions by the totalnumber of positions in the window of comparison (i.e., the window size),and multiplying the result by 100 to yield the percentage of sequenceidentity. Determining the percentage of sequence identity can be donemanually, or by making use of computer programs that are available inthe art.

“Crystal” or “crystalline structure,” as used herein, refers to a solidmaterial, whose constituent atoms, molecules, or ions are arranged in anorderly repeating pattern extending in all three spatial dimensions. Theprocess of forming a crystalline structure from a fluid or frommaterials dissolved in the fluid is often referred to as“crystallization” or “crystallogenesis.” Protein crystals are almostalways grown in solution. The most common approach is to lower thesolubility of its component molecules gradually. Crystal growth insolution is characterized by two steps: nucleation of a microscopiccrystallite (possibly having only 100 molecules), followed by growth ofthat crystallite, ideally to a diffraction-quality crystal.

“X-ray crystallography,” as used herein, is a method of determining thearrangement of atoms within a crystal, in which a beam of X-rays strikesa crystal and diffracts into many specific directions. From the anglesand intensities of these diffracted beams, a crystallographer canproduce a three-dimensional picture of the density of electrons withinthe crystal. From this electron density, the mean positions of the atomsin the crystal can be determined, as well as their chemical bonds, theirdisorder and various other information, as will be known by thoseskilled in the art.

The term “atomic coordinates,” as used herein, refers to a set ofthree-dimensional co-ordinates for atoms within a molecular structure.In one embodiment, atomic-co-ordinates are obtained using X-raycrystallography according to methods well-known to those of ordinarilyskill in the art of biophysics. Briefly described, X-ray diffractionpatterns can be obtained by diffracting X-rays off a crystal. Thediffraction data are used to calculate an electron density map of theunit cell comprising the crystal; the maps are used to establish thepositions of the atoms (i.e., the atomic co-ordinates) within the unitcell. Those skilled in the art understand that a set of structureco-ordinates determined by X-ray crystallography contains standarderrors. In other embodiments, atomic co-ordinates can be obtained usingother experimental biophysical structure determination methods that caninclude electron diffraction (also known as electron crystallography)and nuclear magnetic resonance (NMR) methods. In yet other embodiments,atomic coordinates can be obtained using molecular modeling tools whichcan be based on one or more of ab initio protein folding algorithms,energy minimization, and homology-based modeling. These techniques arewell known to persons of ordinary skill in the biophysical andbioinformatic arts.

“Solving the structure” as used herein refers to determining thearrangement of atoms or the atomic coordinates of a protein, and isoften done by a biophysical method, such as X-ray crystallography.

The term “compound” or “test compound” or “candidate compound” or “drugcandidate compound” as used herein describes any molecule, eithernaturally occurring or synthetic that is tested in an assay, such as ascreening assay or drug discovery assay. As such, these compoundscomprise organic and inorganic compounds. The compounds includepolynucleotides, lipids or hormone analogs that are characterized by lowmolecular weights. Other biopolymeric organic test compounds includesmall peptides or peptide-like molecules (peptidomimetics) comprisingfrom about 2 to about 40 amino acids and larger polypeptides comprisingfrom about 40 to about 500 amino acids, such as antibodies, antibodyfragments or antibody conjugates. Test compounds can also be proteinscaffolds. For high-throughput purposes, test compound libraries may beused, such as combinatorial or randomized libraries that provide asufficient range of diversity. Examples include, but are not limited to,natural compound libraries, allosteric compound libraries, peptidelibraries, antibody fragment libraries, synthetic compound libraries,fragment-based libraries, phage-display libraries, and the like. A moredetailed description can be found further in the specification.

As used herein, the terms “determining,” “measuring,” “assessing,”“monitoring,” and “assaying” are used interchangeably and include bothquantitative and qualitative determinations.

The term “biologically active,” with respect to a GPCR, refers to a GPCRhaving a biochemical function (e.g., a binding function, a signaltransduction function, or an ability to change conformation as a resultof ligand binding) of a naturally occurring GPCR.

The terms “therapeutically effective amount,” “therapeutically effectivedose” and “effective amount,” as used herein, mean the amount needed toachieve the desired result or results.

The term “pharmaceutically acceptable,” as used herein, means a materialthat is not biologically or otherwise undesirable, i.e., the materialmay be administered to an individual along with the compound withoutcausing any undesirable biological effects or interacting in adeleterious manner with any of the other components of thepharmaceutical composition in which it is contained.

DETAILED DESCRIPTION

Despite the great diversity of ligands that may activate GPCRs, theyinteract with a relatively small number of intracellular proteins toinduce profound physiological change. Heterotrimeric G proteins,β-arrestins and GPCR kinases are well known for their ability tospecifically recognize GPCRs in their active state, though poorlyunderstood both from a structural as well as functional point of view.Therefore, surprisingly and advantageously, binding domains wereidentified that specifically bind to GPCR:G protein complexes and arecapable of stabilizing or locking the complex in a functionalconformational state, in particular an active conformational state.Moreover, the binding domains are generic tools for stabilization andcapturing of a GPCR of choice in its G protein-bound state, which isgenerally assumed to represent an active state of a GPCR (as definedherein).

Accordingly, a first aspect of the disclosure relates to a bindingdomain that is directed against and/or specifically binds to a complexcomprising a GPCR and a G protein.

The binding domain of the present disclosure can be any non-naturallyoccurring molecule or part thereof (as defined hereinbefore) that iscapable of specifically binding to a complex comprising a GPCR and a Gprotein. According to a preferred embodiment, the binding domains asdescribed herein are protein scaffolds. The tee n “protein scaffold”refers generally to folding units that form structures, particularlyprotein or peptide structures, that comprise frameworks for the bindingof another molecule, for instance a protein (See, e.g., Skerra (2000),for review). A binding domain can be derived from a naturally occurringmolecule, e.g., from components of the innate or adaptive immune system,or it can be entirely artificially designed. A binding domain can beimmunoglobulin-based or it can be based on domains present in proteins,including but limited to microbial proteins, protease inhibitors,toxins, fibronectin, lipocalins, single chain antiparallel coiled coilproteins or repeat motif proteins. Examples of binding domains which areknown in the art include, but are not limited to: antibodies, heavychain antibodies (hcAb), single domain antibodies (sdAb), minibodies,the variable domain derived from camelid heavy chain antibodies (VHH ornanobodies), the variable domain of the new antigen receptors derivedfrom shark antibodies (VNAR), alphabodies, protein A, protein G,designed ankyrin-repeat domains (DARPins), fibronectin type III repeats,anticalins, knottins, engineered CH2 domains (nanoantibodies), peptidesand proteins, lipopeptides (e.g., pepducins), DNA, and RNA (see, e.g.,Gebauer & Skerra, 2009; Skerra, 2000; Starovasnik et al., 1997; Binz etal., 2004; Koide et al., 1998; Dimitrov, 2009; Nygren et al., 2008;WO2010066740). Frequently, when generating a particular type of bindingdomain using selection methods, combinatorial libraries comprising aconsensus or framework sequence containing randomized potentialinteraction residues are used to screen for binding to a molecule ofinterest, such as a protein.

The binding domain of the present disclosure may be directed againstand/or specifically bind any GPCR:G protein complex of choice. Preferredtarget complexes are complexes of a GPCR and a G protein that occur innature or, alternatively, for example in case of non-naturally occurringvariants (as described further herein) of GPCRs and G protein, complexeswherein the GPCR and the G protein will associate under the appropriatephysiological conditions. It will be understood by the person skilled inthe art that the structural relationship between GPCR and G proteindetermines whether a particular GPCR:G protein complex can be formed,which will be detailed further below for members of the G protein familyand members of the GPCR family.

With “G proteins” are meant the family of guanine nucleotide-bindingproteins involved in transmitting chemical signals outside the cell, andcausing changes inside the cell. G proteins are key molecular componentsin the intracellular signal transduction following ligand binding to theextracellular domain of a GPCR. They are also referred to as“heterotrimeric G proteins,” or “large G proteins.” G proteins consistof three subunits: alpha (α), beta (β), and gamma (γ) and theirclassification is largely based on the identity of their distinct αsubunits, and the nature of the subsequent transduction event. Furtherclassification of G proteins has come from cDNA sequence homologyanalysis. G proteins bind either guanosine diphosphate (GDP) orguanosine triphosphate (GTP), and possess highly homologous guaninenucleotide binding domains and distinct domains for interactions withreceptors and effectors. Different subclasses of Gαproteins, such asGαs, Gαi, Gαq and Gα12, amongst others, signal through distinct pathwaysinvolving second messenger molecules such as cAMP, inositol triphosphate(IP3), diacylglycerol, intracellular Ca²⁺ and RhoA GTPases. Toillustrate this further, the α subunit (39-46 kDa) contains the guaninenucleotide binding site and possesses GTPase activity; the β (37 kDa)and γ (8 kDa) subunits are tightly associated and function as a βγheterodimer. There are 23 types (including some splicing isoforms) of αsubunits, 6 of β, and 11 of γ currently described. The classes of Gprotein and subunits are subscripted: thus, for example, the α subunitof Gs protein (which activates adenylate cyclase) is Gsa; other Gproteins include Gi, which differs from Gs structurally (different typeof α subunit) and inhibits adenylate cyclase. Further examples areprovided in Table 1.

Typically, in nature, G proteins are in a nucleotide-bound form. Morespecifically, G proteins (or at least the α subunit) are bound to eitherGTP or GDP depending on the activation status of a particular GPCR.Agonist binding to a GPCR promotes interactions with the GDP-bound Gαβγheterotrimer leading to the exchange of GDP for GTP on Gα, and thefunctional dissociation of the G protein into Gα-GTP and Gβγ subunits.The separate Gα-GTP and Gβγ subunits can modulate, either independentlyor in parallel, downstream cellular effectors (channels, kinases orother enzymes, see Table 1). The intrinsic GTPase activity of Gγ leadsto hydrolysis of GTP to GDP and the re-association of Gα-GDP and Gβγsubunits, and the termination of signaling. Thus, G proteins serve asregulated molecular switches capable of eliciting bifurcating signalsthrough α and βγ subunit effects. The switch is turned on by thereceptor and it turns itself off within a few seconds, a time sufficientfor considerable amplification of signal transduction.

“G-protein coupled receptors,” or “GPCRs,” as used herein, arepolypeptides that share a common structural motif, having seven regionsof between 22 to 24 hydrophobic amino acids that form seven alphahelices, each of which spans the membrane. Each span is identified bynumber, i.e., transmembrane-1 (TM1), transmembrane-2 (TM2), etc. Thetransmembrane helices are joined by regions of amino acids betweentransmembrane-2 and transmembrane-3, transmembrane-4 andtransmembrane-5, and transmembrane-6 and transmembrane-7 on theexterior, or “extracellular” side, of the cell membrane, referred to as“extracellular” regions 1, 2 and 3 (EC1, EC2 and EC3), respectively. Thetransmembrane helices are also joined by regions of amino acids betweentransmembrane-1 and transmembrane-2, transmembrane-3 andtransmembrane-4, and transmembrane-5 and transmembrane-6 on theinterior, or “intracellular” side, of the cell membrane, referred to as“intracellular” regions 1, 2 and 3 (IC1, IC2 and IC3), respectively. The“carboxy” (“C”) terminus of the receptor lies in the intracellular spacewithin the cell, and the “amino” (“N”) terminus of the receptor lies inthe extracellular space outside of the cell. Any of these regions arereadily identifiable by analysis of the primary amino acid sequence of aGPCR.

GPCR structure and classification is generally well known in the art andfurther discussion of GPCRs may be found in Probst et al., 1992;Marchese et al., 1994; Lagerstrom & Schioth, 2008; Rosenbaum et al.,2009; and the following books: Jurgen Wess (Ed) Structure-FunctionAnalysis of G Protein-Coupled Receptors published by Wiley-Liss (1 stedition; Oct. 15, 1999); Kevin R. Lynch (Ed) Identification andExpression of G Protein-Coupled Receptors published by John Wiley & Sons(March 1998) and Tatsuya Haga (Ed), G Protein-Coupled Receptors,published by CRC Press (Sep. 24, 1999); and Steve Watson (Ed) G-ProteinLinked Receptor Factsbook, published by Academic Press (1st edition;1994). GPCRs can be grouped on the basis of sequence homology intoseveral distinct families. Although all GPCRs have a similararchitecture of seven membrane-spanning α-helices, the differentfamilies within this receptor class show no sequence homology to oneanother, thus suggesting that the similarity of their transmembranedomain structure might define common functional requirements. Acomprehensive view of the GPCR repertoire was possible when the firstdraft of the human genome became available. Fredriksson and colleaguesdivided 802 human GPCRs into families on the basis of phylogeneticcriteria. This showed that most of the human GPCRs can be found in fivemain families, termed Rhodopsin, Adhesion, Secretin, Glutamate,Frizzled/Taste2 (Fredriksson et al., 2003).

Members of the Rhodopsin family (corresponding to class A (Kolakowski,1994) or Class 1 (Foord et al. (2005) in older classification systems)only have small extracellular loops and the interaction of the ligandsoccurs with residues within the transmembrane cleft. This is by far thelargest group (>90% of the GPCRs) and contains receptors for odorants,small molecules such as catecholamines and amines, (neuro)peptides andglycoprotein hormones. Rhodopsin, a representative of this family, isthe first GPCR for which the structure has been solved (Palczewski etal., 2000). β2-AR, the first receptor interacting with a diffusibleligand for which the structure has been solved (Rosenbaum et al., 2007)also belongs to this family. Based on phylogenetic analysis, class BGPCRs or Class 2 (Foord et al., 2005) receptors have recently beensubdivided into two families: adhesion and secretin (Fredriksson et al.,2003). Adhesion and secretin receptors are characterized by a relativelylong amino terminal extracellular domain involved in ligand-binding.Little is known about the orientation of the transmembrane domains, butit is probably quite different from that of rhodopsin. Ligands for theseGPCRs are hormones, such as glucagon, secretin, gonadotropin-releasinghormone and parathyroid hormone. The Glutamate family receptors (Class Cor Class 3 receptors) also have a large extracellular domain, whichfunctions like a “Venus fly trap” since it can open and close with theagonist bound inside. Family members are the metabotropic glutamate, theCa²⁺-sensing and the γ-aminobutyric acid (GABA)-B receptors.

GPCRs include, without limitation, serotonin olfactory receptors,glycoprotein hormone receptors, chemokine receptors, adenosinereceptors, biogenic amine receptors, melanocortin receptors,neuropeptide receptors, chemotactic receptors, somatostatin receptors,opioid receptors, melatonin receptors, calcitonin receptors, PTH/PTHrPreceptors, glucagon receptors, secretin receptors, latrotoxin receptors,metabotropic glutamate receptors, calcium receptors, GABA-B receptors,pheromone receptors, the protease-activated receptors, the rhodopsinsand other G-protein coupled seven transmembrane segment receptors. GPCRsalso include these GPCR receptors associated with each other ashomomeric or heteromeric dimers or as higher-order oligomers. The aminoacid sequences (and the nucleotide sequences of the cDNAs which encodethem) of GPCRs are readily available, for example by reference toGenBank (on the World Wide Web at ncbi.nlm.nih.gov/entrez).

Thus, according to specific embodiments, the present disclosure providesfor binding domains that are directed against and/or specifically bindGPCR:G protein complexes wherein the G protein is selected from thegroup consisting of Gs, Gi, Go, Gt, Ggust, Gz, Golf, Gq, G12 and Gβ. Inone preferred embodiment, the G protein is Gs. In another preferredembodiment, the G protein is Gi. In still another preferred embodiment,the G protein is Gt, more specifically transducin. In correspondencethereto, the GPCR comprised within the complex is selected from thegroup consisting of a Gs coupled receptor, a Gi coupled receptor, a Gocoupled receptor, a Gt coupled receptor, a Ggust coupled receptor, aGolf coupled receptor, a Gq coupled receptor, a G12 coupled receptor anda G13 coupled receptor. In one preferred embodiment, the GPCR is a Gscoupled receptor. In another preferred embodiment, the GPCR is a Gicoupled receptor. In still another preferred embodiment, the GPCR is aGt coupled receptor, more specifically rhodopsin. Particularnon-limiting examples are provided in Table 1.

TABLE 1 Non-limiting examples of the relationship of G protein-coupledreceptors and signaling pathways. G protein Effectors/Signaling family αsubunit pathways Use/Receptors Gi family Gi αi Inhibition ofAcetylcholine M2 & M4 receptors Go αo adenylate cyclase Adenosine A1 &A3 receptors (cAMP ↓) Adrenergic α2A, α2B, & α2C receptors Closing Ca²⁺Apelin receptors channels (Ca²⁺↓) Calcium-sensing receptor ChemokineCXCR4 receptor Dopamine D2, D3, D4 GABAB receptor Glutamate mGluR2,mGluR3, mGluR4, mGluR6, mGluR7, & mGluR8 receptors Histamine H2 & H3 &H4 receptors Melatonin MT1, MT2, & MT3 receptors Muscarinic M2 & M4receptors Opioid δ, κ, μ, & nociceptin receptors Prostaglandin EP1, EP3,FP, & TP receptors Serotonin 5-HT1 & 5-HT5 receptors Gt αt ActivationRhodopsin (transducin) phosphodiesterase 6 (vision) Ggust αgustActivation Taste receptors (gustducin) phosphodiesterase 6 (vision) Gzαz Inhibition adenylate unknown cyclase (cAMP ↓) Gs family Gs αsActivation adenylate 5-HT receptors types 5-HT4 and 5-HT7 cyclase (cAMP↑) ACTH receptor Adenosine receptor types A2a and A2b Argininevasopressin receptor 2 β-adrenergic receptors types β1, β2 and β3Calcitonin receptor Calcitonin gene-related peptide receptorCorticotropin-releasing hormone receptor Dopamine receptors D1-likefamily (D1 and D5) FSH-receptor Gastric inhibitory polypeptide receptorGlucagon receptor Histamine H2 receptor Luteinizinghormone/choriogonadotropin receptor Melanocortin receptor Parathyroidhormone receptor 1 Prostaglandin receptor types D2 and I2 Secretinreceptor Thyrotropin receptor Golf αolf Activation adenylate Olfactoryreceptors cyclase (cAMP ↑) Gq family Gq αq Activation of 5-HT2serotonergic receptors phospholipase C Alpha-1 adrenergic receptor (IP₃↑) Vasopressin type 1 receptor Angiotensin II receptor type 1 Calcitoninreceptor Histamine H1 receptor Metabotropic glutamate receptor, Group IM1, M3, and M5 muscarinic receptors G12/13 family G12 α12 G13 α13 Na⁺/H⁺exchange ↑ βγ subunit βγ Opening K⁺ channels (K⁺ ↑) βγ Adenylate cyclase(cAMP) ↑ or ↓ βγ Phospholipase C (IP₃)

Generally, binding domains of the disclosure will at least bind to thoseforms of GPCR:G protein complexes that are most relevant from abiological and/or therapeutic point of view, as will be clear to theskilled person. It will thus be understood that, depending on thepurpose and application, the GPCR and G protein comprised in the targetcomplex may be naturally occurring or non-naturally occurring (i.e.,altered by man). The term “naturally occurring,” as used herein, means aGPCR or a G protein that are naturally produced. In particular, wildtype polymorphic variants and isoforms of GPCRs and G proteins, as wellas orthologs across different species are examples of naturallyoccurring proteins, and are found for example, and without limitation,in a mammal, more specifically in a human, or in a virus, or in a plant,or in an insect, amongst others). Thus, such GPCRs or G proteins arefound in nature. The term “non-naturally occurring,” as used herein,means a GPCR or a G protein that is not naturally occurring. In certaincircumstances, it may be advantageous that the GPCR and/or G proteincomprised in the complex are non-naturally occurring proteins. Forexample, and for illustration purposes only, to increase the probabilityof obtaining crystals of a GPCR:G protein complex stabilized by thebinding domains of the present invention, it might be desired to performsome protein engineering without or only minimally affecting ligandbinding affinity. Or, alternatively or additionally, to increasecellular expression levels of a GPCR and/or a G protein, or to increasethe stability, one might also consider to introduce certain mutations inthe GPCR and/or the G protein of interest. Non-limiting examples ofnon-naturally occurring GPCRs include, without limitation, GPCRs thathave been made constitutively active through mutation, GPCRs with a loopdeletion, GPCRs with an N- and/or C-terminal deletion, GPCRs with asubstitution, an insertion or addition, or any combination thereof, inrelation to their amino acid or nucleotide sequence, or other variantsof naturally occurring GPCRs. Similarly, non-limiting examples ofnon-naturally occurring G proteins include, without limitation, Gproteins with an N- and/or C-terminal deletion, G proteins with asubstitution, an insertion or addition, or any combination thereof, inrelation to their amino acid or nucleotide sequence, or other variantsof naturally occurring G proteins. Also comprised within the scope ofthe present disclosure are target GPCR:G protein complexes comprising achimeric or hybrid GPCR, for example a chimeric GPCR with an N- and/orC-terminus from one GPCR and loops of a second GPCR, or comprising aGPCR fused to a moiety, such as T4 lysozyme (see also Example section).

According to specific embodiments, a non-naturally occurring GPCR or Gprotein, as comprised in the GPCR:G protein complex, may have an aminoacid sequence that is at least 80% identical to, at least 90% identicalto, at least 95% identical to, at least 97% identical to, or at least99% identical to, a naturally occurring GPCR or G protein. To illustratethis further, and taking the β2-adrenergic receptor as a particularnon-limiting example of a GPCR within the scope hereof, it should beclear from the above that in addition to the human β₂ adrenergicreceptor (e.g., the sequence described by GenBank accession numberNP_000015), the mouse β₂ adrenergic receptor (e.g., as described byGenBank accession no. NM 007420) or other mammalian β₂ adrenergicreceptor are encompassed. Also envisaged are wild-type polymorphicvariants and certain other active variants of the β₂ adrenergic receptorfrom a particular species. For example, “human β₂ adrenergic receptor”has an amino acid sequence that is at least 80% identical to, at least90% identical to, at least 95% identical to, at least 97% identical to,or at least 99% identical to the naturally occurring “human β₂adrenoreceptor” of GenBank accession number NP_000015.

Analogously, and taking Gαs, Gαi and Gαt as particular non-limitingexamples of subunits of G proteins within the scope of the presentinvention, it should be clear from the above that in addition to thehuman Gαs or Gαi or Gαt, the mouse Gαs or Gαi or Gαt proteins or othermammalian Gαs or Gαi or Gαt proteins are encompassed. Also envisaged arewild-type polymorphic variants and certain other active variants of theGαs or Gαi or Gαt from a particular species. For example, a “human Gαs”or a “human Gαi” or a “human Gαt” has an amino acid sequence that is atleast 80% identical to, at least 90% identical to, at least 95%identical to, at least 97% identical to, or at least 99% identical tothe naturally occurring “human Gαs” or “human Gαi” or “human Gαt” ofGenbank accession number P63092, P63096 and P11488, respectively.Further, many isoforms of G protein subunits exist, including forexample isoforms of Gs and Gi proteins (Gαs: GNAS; Gα0: GNAO1; Gαi:GNAI1 or GNAI2 or GNAI3; Gβ: GNB1 or GNB2 or GNB3 or GNB4 or GNB5 orGNB1L or GNB2L; Gγ: GNGT1 or GNGT2 or GNG2 or GNG3 or GNG4 or GNG5 orGNG7 or GNG8 or GNG10 or GNG11 or GNG12 or GNG13; according to HGNCstandardized nomenclature to human genes; accession numbers of differentisoforms from different organisms are available from the World Wide Webat uniprot.org).

Some particular examples of isoforms of G protein subunits are providedin Table 5. The skilled person will appreciate that the amino acidsequences of the different G protein subunits are almost 100%, if not100%, conserved across species and organisms. Notably, sequencealignment of the amino acid sequence of the human, bovine, rat and mouseβ subunit of the G protein reveals that the amino acid sequences betweenthese organisms are 100% conserved. Analogously, the amino acidsequences of the human, mouse, and bovine γ subunit of the G protein are100% identical. The rat and mouse Gαs amino acid sequences are also 100%identical, whereas human and bovine Gαs only differ in 1 or 2 aminoacids, respectively. Thus, the binding domains directed against and/orspecifically binding to a GPCR:G protein complex, and particularlybinding to the G protein comprised in the complex, are expected to becross-reactive. It will also be clear that the GPCR and G proteincomprised in the target complex may be from the same or differentspecies. Preferably, the GPCR and/or the G protein is a mammalianprotein, or a plant protein, or a microbial protein, or a viral protein,or an insect protein. More preferably, the GPCR is a human protein.

It is also expected that binding domains of the disclosure willgenerally be capable of binding to GPCR:G protein complexes comprisingall naturally occurring or synthetic analogs, variants, mutants,alleles, parts, fragments, and isoforms of a particular GPCR and/or Gprotein comprised in the complex; or at least to those analogs,variants, mutants, alleles, parts, fragments, and isoforms of aparticular GPCR and/or G protein comprised in the complex that containone or more antigenic determinants or epitopes that are essentially thesame as the antigenic determinant(s) or epitope(s) to which the bindingdomains of the disclosure bind to a particular GPCR and/or G proteincomprised in the complex.

Various methods may be used to determine specific binding (as definedhereinbefore) between the binding domain and a target GPCR:G proteincomplex, including for example, enzyme linked immunosorbent assays(ELISA), surface Plasmon resonance assays, phage display, and the like,which are common practice in the art, for example, in discussed inSambrook et al. (2001), Molecular Cloning, A Laboratory Manual. ThirdEdition. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.,and are further illustrated in the Example section. It will beappreciated that for this purpose often a unique label or tag will beused, such as a peptide label, a nucleic acid label, a chemical label, afluorescent label, or a radio frequency tag, as described furtherherein.

According to a specific embodiment, the binding domain against a GPCR:Gprotein complex binds with higher affinity to the complex compared withbinding to the G protein alone and/or to the GPCR alone, respectively.In one embodiment, the binding domain against a GPCR:G protein complexspecifically binds to the GPCR comprised in the complex, and not to theG protein. Preferably, in another embodiment, the binding domain againsta GPCR:G protein complex specifically binds to the G protein comprisedin the complex, and not to the GPCR. More specifically, the bindingdomain against a GPCR:G protein complex specifically binds to the Gprotein which is a Gs protein comprised in a complex of a Gs coupledreceptor and a Gs protein.

It is well-known that GPCRs are conformationally complex membraneproteins that exhibit a spectrum of functional behavior in response tonatural and synthetic ligands. In nature, a ligand-bound GPCR mayassociate with a G protein into a complex which will represent aparticular functional conformational state, more specifically an activeconformational state, resulting in a particular biological activity. Thepresent disclosure offers the particular advantage that the bindingdomains as described herein can stabilize various of these activeconformations of GPCRs in complex with a G protein and bound to variousnatural or synthetic ligands. One of skill in the art will recognizethat binding domains that specifically bind to a ligand:GPCR:G proteincomplex, will stabilize the specific conformation of the GPCR comprisedin the complex. In a preferred embodiment, the binding domain is capableof stabilizing, or otherwise, increasing the stability of a particularfunctional conformational state of a GPCR:G protein complex, preferablywherein the GPCR is in an active conformational state. Generally, afunctional conformation state of the GPCR can be a basal conformationalstate, or an active conformational state or an inactive conformationalstate. Preferably, the binding domain of the disclosure is capable ofstabilizing a GPCR in its active conformational state and/or is capableof locking the GPCR in an active conformational state upon binding theGPCR:G protein complex, whether or not in the presence of a receptorligand.

In a particularly preferred embodiment, it is envisaged that the bindingdomain is directed against and/or specifically binds to a complexcomprising a GPCR, a G protein and a receptor ligand (as definedherein). More preferably, the binding domain is directed against and/orspecifically binds to a complex consisting of a GPCR, a G protein and areceptor ligand. A receptor ligand may be a small compound, a peptide,an antibody, or an antibody fragment, and the like, which triggers aresponse upon binding. Receptor ligands, or simply ligands, as usedherein may be orthosteric ligands (both natural and synthetic) that bindto the receptor's active site and are classified according to theirefficacy or in other words to the effect they have on receptor signalingthrough a specific pathway. As used herein, an “agonist” refers to aligand that, by binding a receptor, increases the receptor's signalingactivity. Full agonists are capable of maximal receptor stimulation;partial agonists are unable to elicit full activity even at saturatingconcentrations. Partial agonists can also function as “blockers” bypreventing the binding of more robust agonists. An “antagonist” refersto a ligand that binds a receptor without stimulating any activity. An“antagonist” is also known as a “blocker” because of its ability toprevent binding of other ligands and, therefore, block agonist-inducedactivity. Further, an “inverse agonist” refers to an antagonist that, inaddition to blocking agonist effects, reduces receptors' basal orconstitutive activity below that of the unliganded receptor. Preferably,the binding domain of the disclosure is directed against and/orspecifically binds to a complex comprising a GPCR, a G protein and areceptor ligand, wherein the receptor ligand is an agonist. Morespecifically, the agonist binds the receptor at the orthosteric site.

The signaling activities of GPCRs (and thus their conformationalbehavior) may also be affected by the binding of another kind of ligandsknown as allosteric regulators. “Allosteric regulators” or otherwise“allosteric modulators” or “effector molecules” bind at an allostericsite of a GPCR (that is, a regulatory site physically distinct from theprotein's active site). In contrast to orthosteric ligands, allostericmodulators are non-competitive because they bind receptors at adifferent site and modify receptor function even if the endogenousligand also is binding. Because of this, allosteric modulators are notlimited to simply turning a receptor on or off, the way most drugs are.Instead, they act more like a dimmer switch, offering control over theintensity of activation or deactivation, while allowing the body toretain its natural control over initiating receptor activation, byaltering the affinity of the receptor for its (endogenous) ligand.Allosteric regulators that enhance the protein's activity are referredto herein as “allosteric activators” or “positive allostericmodulators,” whereas those that decrease the protein's activity arereferred to herein as “allosteric inhibitors” or otherwise “negativeallosteric modulators.” Thus, in one particular embodiment, the bindingdomain of the disclosure is directed against and/or specifically bindsto a complex comprising a GPCR, a G protein and a receptor ligand,wherein the receptor ligand is an allosteric modulator, preferably apositive allosteric modulator. More specifically, the positiveallosteric modulator binds the receptor at an allosteric site.

As explained, the canonical view of how GPCRs regulate cellularphysiology is that the binding of ligands (such as hormones,neurotransmitters or sensory stimuli) stabilizes an activeconformational state of the receptor, thereby allowing interactions withheterotrimeric G proteins. In addition to interacting with G proteins,agonist-bound GPCRs associate with GPCR kinases (GRKs), leading toreceptor phosphorylation. A common outcome of GPCR phosphorylation byGRKs is a decrease in GPCR interactions with G proteins and an increasein GPCR interactions with arrestins, which sterically interdict furtherG protein signaling, resulting in receptor desensitization. Asβ-arrestins turn off G protein signals, they can simultaneously initiatea second, parallel set of signal cascades, such as the MAPK pathway.GPCRs also associate with various proteins outside the families ofgeneral GPCR-interacting proteins (G proteins, GRKs, arrestins and otherreceptors). These GPCR-selective partners can mediate GPCR signaling,organize GPCR signaling through G proteins, direct GPCR trafficking,anchor GPCRs in particular subcellular areas and/or influence GPCRpharmacology (Ritter and Hall 2009). In this regard, ligands as usedherein may also be “biased ligands” with the ability to selectivelystimulate a subset of a receptor's signaling activities, for example theselective activation of G protein or β-arrestin function. Such ligandsare known as “biased ligands,” “biased agonists,” or “functionallyselective agonists.” More particularly, ligand bias can be an imperfectbias characterized by a ligand stimulation of multiple receptoractivities with different relative efficacies for different signals(non-absolute selectivity) or can be a perfect bias characterized by aligand stimulation of one receptor activity without any stimulation ofanother known receptor activity. Thus, in one particular embodiment, thebinding domain of the disclosure is directed against and/or specificallybinds to a complex comprising a GPCR, a G protein and a receptor ligand,wherein the receptor ligand is a biased ligand.

Further, according to a preferred embodiment, it is particularlyenvisaged that the binding domain of the disclosure directed againstand/or specifically binding to a GPCR:G protein complex, as describedhereinbefore, is derived from an innate or adaptive immune system.Preferably, the binding domain is derived from an immunoglobulin.Preferably, the binding domain according to the disclosure is anantibody or an antibody fragment. The term “antibody” (Ab) refersgenerally to a polypeptide encoded by an immunoglobulin gene, or afunctional fragment thereof, that specifically binds and recognizes anantigen, and is known to the person skilled in the art. A conventionalimmunoglobulin (antibody) structural unit comprises a tetramer. Eachtetramer is composed of two identical pairs of polypeptide chains, eachpair having one “light” (about 25 kDa) and one “heavy” chain (about50-70 kDa). The N-terminus of each chain defines a variable region ofabout 100 to 110 or more amino acids primarily responsible for antigenrecognition. The terms variable light chain (VL) and variable heavychain (VH) refer to these light and heavy chains respectively. The term“antibody” is meant to include whole antibodies, including single-chainwhole antibodies, and antigen-binding fragments. In some embodiments,antigen-binding fragments may be antigen-binding antibody fragments thatinclude, but are not limited to, Fab, Fab′ and F(ab′)2, Fd, single-chainFvs (scFv), single-chain antibodies, disulfide-linked Fvs (dsFv) andfragments comprising or consisting of either a VL or VH domain, and anycombination of those or any other functional portion of animmunoglobulin peptide capable of binding to the target antigen. Theterm “antibodies” is also meant to include heavy chain antibodies, orfunctional fragments thereof, such as single domain antibodies, morespecifically, immunoglobulin single variable domains such as VHHs ornanobodies, as defined further herein.

Preferably, the binding domain of the disclosure is an immunoglobulinsingle variable domain. More preferably, the binding domain is animmunoglobulin single variable domain that comprises an amino acidsequence comprising four framework regions (FR1 to FR4) and threecomplementarity-determining regions (CDR1 to CDR3), preferably accordingto the following formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1),or any suitable fragment thereof (which will then usually contain atleast some of the amino acid residues that form at least one of thecomplementarity-determining regions).

Binding domains comprising 4 FRs and 3 CDRs are known to the personskilled in the art and have been described, as a non-limiting example,in Wesolowski et al. (2009, Med. Microbiol. Immunol. 198:157). Typical,but non-limiting, examples of immunoglobulin single variable domainsinclude light chain variable domain sequences (e.g., a V_(L) domainsequence), or heavy chain variable domain sequences (e.g., a V_(H)domain sequence), which are usually derived from conventional four-chainantibodies. Preferably, the immunoglobulin single variable domains arederived from camelid antibodies, preferably from heavy chain camelidantibodies, devoid of light chains, and are known as V_(H)H domainsequences or nanobodies (as described further herein).

The term “nanobody” (Nb), as used herein, refers to the smallest antigenbinding fragment or single variable domain (V_(H)H) derived fromnaturally occurring heavy chain antibody and is known to the personskilled in the art. They are derived from heavy chain only antibodies,seen in camelids (Hamers-Casterman et al., 1993; Desmyter et al., 1996).In the family of “camelids” immunoglobulins devoid of light polypeptidechains are found. “Camelids” comprise old world camelids (Camelusbactrianus and Camelus dromedarius) and new world camelids (for example,Lama paccos, Lama glama, Lama guanicoe and Lama vicugna). The singlevariable domain heavy chain antibody is herein designated as a Nanobodyor a V_(H)H antibody. Nanobody™ and Nanobodies™ are trademarks of AblynxNV (Belgium). The small size and unique biophysical properties of Nbsexcel conventional antibody fragments for the recognition of uncommon orhidden epitopes and for binding into cavities or active sites of proteintargets. Further, Nbs can be designed as multispecific and/ormultivalent antibodies or attached to reporter molecules (Conrath etal., 2001). Nbs are stable and rigid single domain proteins that caneasily be manufactured and survive the gastro-intestinal system.Therefore, Nbs can be used in many applications including drug discoveryand therapy (Saerens et al., 2008) but also as a versatile and valuabletool for purification, functional study and crystallization of proteins(Conrath et al., 2009). A particular class of nanobodies that act ascrystallization chaperones binding conformational epitopes of nativetargets are called XAPERONES™ and are particularly envisaged here.XAPERONES™ are unique tools in structural biology. XAPERONE™ is atrademark of VIB and VUB (Belgium). Major advantages for the use ofcamelid antibody fragments as crystallization aid are that XAPERONES™(1) bind cryptic epitopes and lock proteins in unique nativeconformations, (2) increase the stability of soluble proteins andsolubilized membrane proteins, (3) reduce the confoimational complexityof soluble proteins and solubilized membrane proteins, (4) increase thepolar surface enabling the growth of diffracting crystals, (5) sequesteraggregative or polymerizing surfaces, (6) allow to affinity-trap activeprotein.

Thus, the immunoglobulin single variable domains hereof, in particularthe nanobodies hereof, generally comprise a single amino acid chain thattypically comprises 4 “framework sequences” or FRs and three“complementarity-deter mining regions” or CDRs according to formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1).

The term “complementarity-determining region” or “CDR” refers tovariable regions in immunoglobulin single variable domains and containsthe amino acid sequences capable of specifically binding to antigenictargets. These CDR regions account for the basic specificity of thenanobody for a particular antigenic determinant structure. Such regionsare also referred to as “hypervariable regions.” The immunoglobulinsingle variable domains have 3 CDR regions, each non-contiguous with theothers (termed CDR1, CDR2, CDR3). It should be clear that frameworkregions of immunoglobulin single variable domains may also contribute tothe binding of their antigens (Desmyter et al., 2002; Korotkov et al.,2009). Non-limiting examples of such immunoglobulin single variabledomains according to the present disclosure as well as particularcombinations of FRs and CDRs are as described herein (see Tables 2 and3). The delineation of the CDR sequences (and thus also the FRsequences) is based on the IMGT unique numbering system for V-domainsand V-like domains (Lefranc et al., 2003). Alternatively, thedelineation of the FR and CDR sequences can be done by using the Kabatnumbering system as applied to V_(H)H domains from Camelids in thearticle of Riechmann and Muyldermans (2000). As will be known by theperson skilled in the art, the immunoglobulin single variable domains,in particular the nanobodies, can in particular be characterized by thepresence of one or more Camelidae hallmark residues in one or more ofthe framework sequences (according to Kabat numbering), as described forexample in WO 08/020079, on page 75, Table A-3, incorporated herein byreference).

In a preferred embodiment, the disclosure provides immunoglobulin singlevariable domains with an amino acid sequence selected from the groupconsisting of amino acid sequences that essentially consist of fourframework regions (FR1 to FR4, respectively) and threecomplementarity-determining regions (CDR1 to CDR3, respectively), inwhich the CDR sequences of the amino acid sequences have at least 70%amino acid identity, preferably at least 80% amino acid identity, morepreferably at least 90% amino acid identity, such as at least 95%, atleast 96%, at least 97%, at least 98%, at least 99%, or even 100% aminoacid identity with the CDR sequences (see Table 3) of at least one ofthe immunoglobulin single variable domains of SEQ ID NO:s 1-6,preferably SEQ ID NO: 1 and/or 4. It will be understood that fordetermining the degree of amino acid identity of the amino acidsequences of the CDRs of one or more sequences of the immunoglobulinsingle variable domains, the amino acid residues that from the frameworkregions are disregarded. Some preferred, but non-limiting, examples ofimmunoglobulin single variable domains of the disclosure are given inSEQ ID NO:s 1 to 6, preferably SEQ ID NO: 1 and/or SEQ ID NO: 4 (seeTable 2).

It should be noted that the immunoglobulin single variable domains, inparticular the nanobodies, of the disclosure in their broadest sense arenot limited to a specific biological source or to a specific method ofpreparation. For example, the immunoglobulin single variable domainshereof, in particular the nanobodies, can generally be obtained: (1) byisolating the V_(H)H domain of a naturally occurring heavy chainantibody; (2) by expression of a nucleotide sequence encoding anaturally occurring V_(H)H domain; (3) by “humanization” of a naturallyoccurring V_(H)H domain or by expression of a nucleic acid encoding asuch humanized V_(H)H domain; (4) by “camelization” of a naturallyoccurring VH domain from any animal species, and in particular from amammalian species, such as from a human being, or by expression of anucleic acid encoding such a camelized VH domain; (5) by “camelization”of a “domain antibody” or “Dab” as described in the art, or byexpression of a nucleic acid encoding such a camelized VII domain; (6)by using synthetic or semi-synthetic techniques for preparing proteins,polypeptides or other amino acid sequences known per se; (7) bypreparing a nucleic acid encoding a nanobody using techniques fornucleic acid synthesis known per se, followed by expression of thenucleic acid thus obtained; and/or (8) by any combination of one or moreof the foregoing.

One preferred class of immunoglobulin single variable domainscorresponds to the V_(H)H domains of naturally occurring heavy chainantibodies directed against a target complex of a GPCR and a G protein.Although naive or synthetic libraries of immunoglobulin single variabledomains may contain conformational binders against the target complex, apreferred embodiment of this disclosure includes the immunization of aCamelidae with a target complex to expose the immune system of theanimal with the conformational epitopes that are unique to the complex.Animals can be immunized with mixtures of the interacting monomers.Optionally, the complex can be stabilized by chemical cross-linking orby adding cooperative/allosteric ligands/metabolites that stabilize thecomplex (orthosteric agonists, allosteric activators, Ca⁺⁺, ATP, etc.).The complex can also be stabilized by covalent modification(phosphorylation, etc.) of (one of) the members of the complex. In analternatively embodiment, one might also immunize Camelidae with theGPCR and/or the G protein alone, thus not in complex with each other.Optionally, the GPCR and/or the G protein may also be stabilized, forexample, by adding cooperative/allosteric ligands/metabolites thatstabilize the GPCR and/or G protein (orthosteric agonists, allostericactivators, Ca⁺⁺, ATP, amongst others).

Thus, such V_(H)H sequences can generally be generated or obtained bysuitably immunizing a species of Camelid with a target complexcomprising a GPCR and a G protein, or with either one or both of itsconstituting member proteins (i.e., so as to raise an immune responseand/or heavy chain antibodies directed against a target complex), byobtaining a suitable biological sample from the Camelid (such as a bloodsample, or any sample of B-cells), and by generating V_(H)H sequencesdirected against a target complex, starting from the sample, using anysuitable technique known per se. Such techniques will be clear to theskilled person. Alternatively, such naturally occurring V_(H)H domainscan be obtained from naive libraries of Camelid V_(H)H sequences, forexample by screening such a library using a target complex or at leastone part, fragment, antigenic determinant or epitope thereof using oneor more screening techniques known per se. Such libraries and techniquesare for example described in WO9937681, WO0190190, WO03025020 andWO03035694. Alternatively, improved synthetic or semi-syntheticlibraries derived from naive V_(H)H libraries may be used, such asV_(H)H libraries obtained from naive V_(H)H libraries by techniques suchas random mutagenesis and/or CDR shuffling, as for example described inWO0043507. Yet another technique for obtaining V_(H)H sequences directedagainst a target involves suitably immunizing a transgenic mammal thatis capable of expressing heavy chain antibodies (i.e., so as to raise animmune response and/or heavy chain antibodies directed against atarget), obtaining a suitable biological sample from the transgenicmammal (such as a blood sample, or any sample of B-cells), and thengenerating V_(H)H sequences directed against a target starting from thesample, using any suitable technique known per se. For example, for thispurpose, the heavy chain antibody-expressing mice and the furthermethods and techniques described in WO02085945 and in WO04049794 can beused.

A particularly preferred class of immunoglobulin single variable domainshereof, in particular nanobodies hereof, comprises immunoglobulin singlevariable domains with an amino acid sequence that corresponds to theamino acid sequence of a naturally occurring V_(H)H domain, but that hasbeen “humanized,” i.e., by replacing one or more amino acid residues inthe amino acid sequence of the naturally occurring V_(H)H sequence (andin particular in the framework sequences) by one or more of the aminoacid residues that occur at the corresponding position(s) in a VH domainfrom a conventional 4-chain antibody from a human being. This can beperformed in a manner known per se, which will be clear to the skilledperson, for example on the basis of the further description herein andthe prior art on humanization referred to herein. Again, it should benoted that such humanized immunoglobulin single variable domains of thedisclosure can be obtained in any suitable manner known per se (i.e., asindicated under points (1)-(8) above) and thus are not strictly limitedto polypeptides that have been obtained using a polypeptide thatcomprises a naturally occurring V_(H)H domain as a starting material.Humanized immunoglobulin single variable domains, in particularnanobodies, may have several advantages, such as a reducedimmunogenicity, compared to the corresponding naturally occurring V_(H)Hdomains. Such humanization generally involves replacing one or moreamino acid residues in the sequence of a naturally occurring V_(H)H withthe amino acid residues that occur at the same position in a human VHdomain, such as a human VH3 domain. The humanizing substitutions shouldbe chosen such that the resulting humanized immunoglobulin singlevariable domains still retain the favorable properties of immunoglobulinsingle variable domains as defined herein. The skilled person will beable to select humanizing substitutions or suitable combinations ofhumanizing substitutions which optimize or achieve a desired or suitablebalance between the favorable properties provided by the humanizingsubstitutions on the one hand and the favorable properties of naturallyoccurring V_(H)H domains on the other hand.

Another particularly preferred class of immunoglobulin single variabledomains hereof, in particular nanobodies hereof, comprisesimmunoglobulin single variable domains with an amino acid sequence thatcorresponds to the amino acid sequence of a naturally occurring VHdomain, but that has been “camelized,” i.e., by replacing one or moreamino acid residues in the amino acid sequence of a naturally occurringVH domain from a conventional 4-chain antibody by one or more of theamino acid residues that occur at the corresponding position(s) in aV_(H)H domain of a heavy chain antibody. Such “camelizing” substitutionsare preferably inserted at amino acid positions that form and/or arepresent at the VH-VL interface, and/or at the so-called Camelidaehallmark residues, as defined herein (see for example WO9404678).Preferably, the VH sequence that is used as a starting material orstarting point for generating or designing the camelized nanobody ispreferably a VH sequence from a mammal, more preferably the VH sequenceof a human being, such as a VH3 sequence. However, it should be notedthat such camelized immunoglobulin single variable domains of thedisclosure can be obtained in any suitable manner known per se (i.e., asindicated under points (1)-(8) above) and thus are not strictly limitedto polypeptides that have been obtained using a polypeptide thatcomprises a naturally occurring VH domain as a starting material.

For example both “humanization” and “camelization” can be performed byproviding a nucleotide sequence that encodes a naturally occurringV_(H)H domain or VH domain, respectively, and then changing, in a mannerknown per se, one or more codons in the nucleotide sequence in such away that the new nucleotide sequence encodes a “humanized” or“camelized” immunoglobulin single variable domains hereof, respectively.This nucleic acid can then be expressed in a manner known per se, so asto provide the desired immunoglobulin single variable domains hereof.Alternatively, based on the amino acid sequence of a naturally occurringV_(H)H domain or VH domain, respectively, the amino acid sequence of thedesired humanized or camelized immunoglobulin single variable domainshereof, respectively, can be designed and then synthesized de novo usingtechniques for peptide synthesis known per se. Also, based on the aminoacid sequence or nucleotide sequence of a naturally occurring V_(H)Hdomain or VH domain, respectively, a nucleotide sequence encoding thedesired humanized or camelized immunoglobulin single variable domainshereof, respectively, can be designed and then synthesized de novo usingtechniques for nucleic acid synthesis known per se, after which thenucleic acid thus obtained can be expressed in a manner known per se, soas to provide the desired immunoglobulin single variable domains hereof.Other suitable methods and techniques for obtaining the immunoglobulinsingle variable domains of the disclosure and/or nucleic acids encodingthe same, starting from naturally occurring VH sequences or preferablyV_(H)H sequences, will be clear from the skilled person, and may forexample comprise combining one or more parts of one or more naturallyoccurring VH sequences (such as one or more FR sequences and/or CDRsequences), one or more parts of one or more naturally occurring V_(H)Hsequences (such as one or more FR sequences or CDR sequences), and/orone or more synthetic or semi-synthetic sequences, in a suitable manner,so as to provide a nanobody of the disclosure or a nucleotide sequenceor nucleic acid encoding the same.

Also within the scope of the disclosure are natural or syntheticanalogs, mutants, variants, alleles, parts or fragments (hereincollectively referred to as “variants”) of the immunoglobulin singlevariable domains, in particular the nanobodies, of the disclosure asdefined herein, and in particular variants of the immunoglobulin singlevariable domains of SEQ ID NOs: 1-6 (see Tables 2-3). Thus, according toone embodiment hereof, the term “immunoglobulin single variable domainhereof” or “nanobody hereof” in its broadest sense also covers suchvariants. Generally, in such variants, one or more amino acid residuesmay have been replaced, deleted and/or added, compared to theimmunoglobulin single variable domains of the disclosure as definedherein. Such substitutions, insertions or deletions may be made in oneor more of the FRs and/or in one or more of the CDRs and, in particular,variants of the FRs and CDRs of the immunoglobulin single variabledomains of SEQ ID NOS:1-6 (see Tables 2 and 3). Variants, as usedherein, are sequences wherein each or any framework region and each orany complementarity-determining region shows at least 80% identity,preferably at least 85% identity, more preferably 90% identity, evenmore preferably 95% identity or, still even more preferably 99% identitywith the corresponding region in the reference sequence (i.e.,FR1_variant versus FR1_reference, CDR1_variant versus CDR1_reference,FR2_variant versus FR2_reference, CDR2_variant versus CDR2_reference,FR3_variant versus FR3_reference, CDR3_variant versus CDR3_reference,FR4_variant versus FR4_reference), as can be measured electronically bymaking use of algorithms such as PILEUP and BLAST (50, 51). Software forperforming BLAST analyses is publicly available through the NationalCenter for Biotechnology Information (on the World Wide Web atncbi.nlm.nih.gov). It will be understood that for determining the degreeof amino acid identity of the amino acid sequences of the CDRs of one ormore sequences of the immunoglobulin single variable domains, the aminoacid residues that form the framework regions are disregarded.Similarly, for determining the degree of amino acid identity of theamino acid sequences of the FRs of one or more sequences of theimmunoglobulin single variable domains hereof, the amino acid residuesthat form the complementarity regions are disregarded. Such variants ofimmunoglobulin single variable domains may be of particular advantagesince they may have improved potency/affinity.

By means of non-limiting examples, a substitution may for example be aconservative substitution (as described herein) and/or an amino acidresidue may be replaced by another amino acid residue that naturallyoccurs at the same position in another V_(H)H domain. Thus, any one ormore substitutions, deletions or insertions, or any combination thereof,that either improve the properties of the immunoglobulin single variabledomains of the disclosure or that at least do not detract too much fromthe desired properties or from the balance or combination of desiredproperties of the nanobody of the disclosure (i.e., to the extent thatthe immunoglobulin single variable domains is no longer suited for itsintended use) are included within the scope hereof. A skilled personwill generally be able to determine and select suitable substitutions,deletions or insertions, or suitable combinations of thereof, based onthe disclosure herein and optionally after a limited degree of routineexperimentation, which may for example involve introducing a limitednumber of possible substitutions and determining their influence on theproperties of the immunoglobulin single variable domains thus obtained.

According to particularly preferred embodiments, variants of theimmunoglobulin single variable domains, in particular the nanobodies, ofthe present disclosure may have a substitution, deletion or insertion,of 1, 2 or 3 amino acids in either one, two or three of the CDRs, morespecifically a substitution, deletion or insertion of 1, 2 or 3 aminoacids (i) in CDR1 or CDR2 or CDR3; (ii) in CDR1 and CDR2, or, in CDR1and CDR3, or, in CDR2 and CDR3; (iii) in CDR1 and CDR2 and CDR3, ofwhich the amino acid sequences of CDRs are as listed in Table 3. Morepreferably, variants of the immunoglobulin single variable domains, inparticular the nanobodies, of the present disclosure may have aconservative substitution (as defined herein) of 1, 2 or 3 amino acidsin one, two or three of the CDRs, more specifically (i) in CDR1 or CDR2or CDR3; (ii) in CDR1 and CDR2, or, in CDR1 and CDR3, or, in CDR2 andCDR3; (iii) in CDR1 and CDR2 and CDR3, as listed in Table 3.

According to a specific embodiment, the present disclosure provides foran immunoglobulin single variable domain comprising an amino acidsequence that comprises four framework regions (FR1 to FR4) and threecomplementarity-determining regions (CDR1 to CDR3), according to thefollowing formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1);

and wherein CDR1 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 13-18,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 13-18,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 13-18,

and wherein CDR2 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 25-30,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 25-30,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 25-30,

and wherein CDR3 is chosen from the group consisting of:

-   -   a) SEQ ID NOs: 37-42,    -   b) Polypeptides that have at least 80% amino acid identity with        SEQ ID NOs: 37-42,    -   c) Polypeptides that have 3, 2 or 1 amino acid difference with        SEQ ID NOs: 37-42.

In a particularly preferred embodiment, the present disclosure providesfor an immunoglobulin single variable domain comprising an amino acidsequence that comprises four framework regions (FR1 to FR4) and threecomplementarity-determining regions (CDR1 to CDR3), according to thefollowing formula (1):FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (1);wherein CDR1 is SEQ ID NO: 13; wherein CDR2 is SEQ ID NO: 25; andwherein CDR3 is SEQ ID NO: 37.

Further, and depending on the host organism used to express the bindingdomain, in particular the immunoglobulin single variable domain hereof,deletions and/or substitutions may be designed in such a way that one ormore sites for post-translational modification (such as one or moreglycosylation sites) are removed, as will be within the ability of theperson skilled in the art. Alternatively, substitutions or insertionsmay be designed so as to introduce one or more sites for attachment offunctional groups (as described herein), for example to allowsite-specific pegylation.

Examples of modifications, as well as examples of amino acid residueswithin the binding domain, in particular the immunoglobulin singlevariable domain of the disclosure that can be modified (i.e., either onthe protein backbone but preferably on a side chain), methods andtechniques that can be used to introduce such modifications and thepotential uses and advantages of such modifications will be clear to theskilled person. For example, such a modification may involve theintroduction (e.g., by covalent linking or in another suitable manner)of one or more functional groups, residues or moieties into or onto thebinding domain, in particular the immunoglobulin single variable domainhereof, and in particular of one or more functional groups, residues ormoieties that confer one or more desired properties or functionalitiesto the binding domain hereof.

Examples of such functional groups and of techniques for introducingthem will be clear to the skilled person, and can generally comprise allfunctional groups and techniques mentioned in the art as well as thefunctional groups and techniques known per se for the modification ofpharmaceutical proteins, and in particular for the modification ofantibodies or antibody fragments (including ScFvs and single domainantibodies), for which reference is for example made to Remington'sPharmaceutical Sciences, 16th ed., Mack Publishing Co., Easton, Pa.(1980). Such functional groups may for example be linked directly (forexample covalently) to the binding domain, in particular theimmunoglobulin single variable domain hereof, or optionally via asuitable linker or spacer, as will again be clear to the skilled person.

One of the most widely used techniques for increasing the half-lifeand/or reducing immunogenicity of pharmaceutical proteins comprisesattachment of a suitable pharmacologically acceptable polymer, such aspoly(ethylene glycol) (PEG) or derivatives thereof (such asmethoxypoly(ethylene glycol) or mPEG). Generally, any suitable form ofpegylation can be used, such as the pegylation used in the art forantibodies and antibody fragments (including but not limited to (single)domain antibodies and ScFvs); reference is made to for example Chapman,Nat. Biotechnol., 54, 531-545 (2002); by Veronese and Harris, Adv. DrugDeliv. Rev. 54, 453-456 (2003), by Harris and Chess, Nat. Rev. Drug.Discov., 2, (2003) and in WO04060965. Various reagents for pegylation ofproteins are also commercially available, for example from NektarTherapeutics, USA. Preferably, site-directed pegylation is used, inparticular via a cysteine-residue (see for example Yang et al., ProteinEngineering, 16, 10, 761-770 (2003). For example, for this purpose, PEGmay be attached to a cysteine residue that naturally occurs in aimmunoglobulin single variable domain hereof, a immunoglobulin singlevariable domain of the disclosure may be modified so as to suitablyintroduce one or more cysteine residues for attachment of PEG, or anamino acid sequence comprising one or more cysteine residues forattachment of PEG may be fused to the N- and/or C-terminus of a nanobodyhereof, all using techniques of protein engineering known per se to theskilled person. Preferably, for the immunoglobulin single variabledomain hereof, a PEG is used with a molecular weight of more than 5000,such as more than 10,000 and less than 200,000, such as less than100,000; for example in the range of 20,000-80,000.

Another, usually less preferred modification comprises N-linked orO-linked glycosylation, usually as part of co-translational and/orpost-translational modification, depending on the host cell used forexpressing the binding domain, in particular the immunoglobulin singlevariable domain hereof. Another technique for increasing the half-lifeof a binding domain may comprise the engineering into bifunctionalbinding domains (for example, one immunoglobulin single variable domainagainst the target GPCR:G protein complex and one against a serumprotein such as albumin) or into fusions of binding domains, inparticular immunoglobulin single variable domains, with peptides (forexample, a peptide against a serum protein such as albumin).

Yet another modification may comprise the introduction of one or moredetectable labels or other signal-generating groups or moieties,depending on the intended use of the labeled binding domain. Suitablelabels and techniques for attaching, using and detecting them will beclear to the skilled person, and for example include, but are notlimited to, fluorescent labels (such as fluorescein, isothiocyanate,rhodamine, phycoerythrin, phycocyanin, allophycocyanin, o-phthaldehyde,and fluorescamine and fluorescent metals such as Eu or others metalsfrom the lanthanide series), phosphorescent labels, chemiluminescentlabels or bioluminescent labels (such as luminal, isoluminol, theromaticacridinium ester, imidazole, acridinium salts, oxalate ester, dioxetaneor GFP and its analogs), radio-isotopes, metals, metals chelates ormetallic cations or other metals or metallic cations that areparticularly suited for use in in vivo, in vitro or in situ diagnosisand imaging, as well as chromophores and enzymes (such as malatedehydrogenase, staphylococcal nuclease, delta-V-steroid isomerase, yeastalcohol dehydrogenase, alpha-glycerophosphate dehydrogenase, triosephosphate isomerase, biotinavidin peroxidase, horseradish peroxidase,alkaline phosphatase, asparaginase, glucose oxidase, beta-galactosidase,ribonuclease, urease, catalase, glucose-VI-phosphate dehydrogenase,glucoamylase and acetylcholine esterase).

Other suitable labels will be clear to the skilled person, and forexample include moieties that can be detected using NMR or ESRspectroscopy. Such labeled binding domains of the disclosure may forexample be used for in vitro, in vivo or in situ assays (includingimmunoassays known per se such as ELISA, RIA, EIA and other “sandwichassays,” etc.) as well as in vivo diagnostic and imaging purposes,depending on the choice of the specific label. As will be clear to theskilled person, another modification may involve the introduction of achelating group, for example to chelate one of the metals or metalliccations referred to above. Suitable chelating groups for exampleinclude, without limitation, diethyl-enetriaminepentaacetic acid (DTPA)or ethylenediaminetetraacetic acid (EDTA).

Yet another modification may comprise the introduction of a functionalgroup that is one part of a specific binding pair, such as thebiotin-(strept)avidin binding pair. Such a functional group may be usedto link the binding domain of the disclosure to another protein,polypeptide or chemical compound that is bound to the other half of thebinding pair, i.e., through formation of the binding pair. For example,a immunoglobulin single variable domain of the disclosure may beconjugated to biotin, and linked to another protein, polypeptide,compound or carrier conjugated to avidin or streptavidin. For example,such a conjugated immunoglobulin single variable domain may be used as areporter, for example in a diagnostic system where a detectablesignal-producing agent is conjugated to avidin or streptavidin. Suchbinding pairs may for example also be used to bind the immunoglobulinsingle variable domain of the disclosure to a carrier, includingcarriers suitable for pharmaceutical purposes. One non-limiting exampleare the liposomal formulations described by Cao and Suresh, Journal ofDrug Targetting, 8, 4, 257 (2000). Such binding pairs may also be usedto link a therapeutically active agent to the binding domain hereof.

Also encompassed within the scope of the present disclosure are thebinding domains, in particular the immunoglobulin single variabledomains of the disclosure that are in a “multivalent” form and areformed by bonding, chemically or by recombinant DNA techniques, togethertwo or more monovalent immunoglobulin single variable domains.Non-limiting examples of multivalent constructs include “bivalent”constructs, “trivalent” constructs, “tetravalent” constructs, and so on.The immunoglobulin single variable domains comprised within amultivalent construct may be identical or different. In anotherparticular embodiment, the immunoglobulin single variable domains of thedisclosure are in a “multi-specific” form and are formed by bondingtogether two or more immunoglobulin single variable domains, of which atleast one with a different specificity. Non-limiting examples ofmulti-specific constructs include “bi-specific” constructs,“tri-specific” constructs, “tetra-specific” constructs, and so on. Toillustrate this further, any multivalent or multispecific (as definedherein) immunoglobulin single variable domain of the disclosure may besuitably directed against two or more different epitopes on the sameantigen, for example against two or more different parts of the Gprotein as comprised in the GPCR:G protein complex; or may be directedagainst two or more different antigens, for example against an epitopeof the GPCR and an epitope of the G protein. In particular, a monovalentimmunoglobulin single variable domain of the disclosure is such that itwill bind to the target GPCR:G protein complex (as described herein)with an affinity less than 500 nM, preferably less than 200 nM, morepreferably less than 10 nM, such as less than 500 pM. Multivalent ormultispecific immunoglobulin single variable domains of the disclosuremay also have (or be engineered and/or selected for) increased avidityand/or improved selectivity for the desired GPCR:G protein complex,and/or for any other desired property or combination of desiredproperties that may be obtained by the use of such multivalent ormultispecific immunoglobulin single variable domains.

Further, the binding domain hereof, in particular the immunoglobulinsingle variable domain hereof, may generally be directed against orspecifically binding to any conformational epitope that is representedby or accessible on or part of a complex comprising a GPCR and a Gprotein. A binding domain that specifically binds to a “threedimensional” epitope or “conformational” epitope is a binding domainthat specifically binds to a tertiary or quaternary structure of afolded protein or protein complex. Such a binding domain binds at muchreduced (i.e., by a factor of at least 2, 5, 10, 50 or 100) affinity tothe linear (i.e., unfolded, denatured) polypeptide chain. The structureto which such a binding domain binds usually contains amino acids thatare discontinuous in the linear sequence of the protein (complex). Inother words, binding of such a binding domain to a polypeptide isdependent upon the polypeptide being folded into a particular threedimensional conformation. It should be clear that the conformationalepitopes selectively recognized by the binding domains of the disclosurecan be GPCR specific epitopes, or G protein specific epitopes, orotherwise GPCR:G protein complex-specific epitopes, which are onlyformed upon association of the constituting proteins, and thus bycombining amino acid residues of the GPCR and the G protein. In oneembodiment, the binding domain hereof, in particular the immunoglobulinsingle variable domain, specifically binds to any conformational epitopeof any desired G protein or parts thereof. In another embodiment, theconformational epitope can be part of an intracellular or extracellularregion, or an intramembraneous region, or a domain or loop structure ofany desired GPCR. It is clear that some of these conformational epitopeswill be accessible in the GPCR and/or the G protein in thenon-associated form, while others will only be accessible once thecomplex is formed. According to one specific embodiment, the bindingdomain hereof, in particular the immunoglobulin single variable domain,specifically binds a conformational epitope at the interface between theα and β subunit of a G protein, as is specified as a non-limitingexample further herein (see Example section).

According to other specific embodiments, the binding domain binds to aGPCR:G protein complex wherein the G protein is in its nucleotide freefrom. According to further specific embodiments, the binding domainhereof, in particular the immunoglobulin single variable domain,inhibits or prevents the dissociation of the GPCR:G protein in thepresence of nucleotides, in particular guanine nucleotides, such as GDPor GTP, or analogs thereof, such as nonhydrolyzable GTP analogs, such asGTPγS, or GDP in combination with aluminum or beryllium fluoridespecies, or minimal nucleotide analogs such as pyrophosphate orforcarnet. In the absence of the binding domains hereof, dissociation ofthe GPCR:G protein complex normally occurs in the presence of thesenucleotides.

According to one particular aspect, the disclosure also provides for abinding domain that is directed against or specifically binds to a Gprotein (i.e., a G protein alone, thus not in complex with a GPCR). Inspecific embodiments, the binding domain as described herein is directedagainst and/or specifically binds to a Gs protein. According to specificembodiments of this aspect, the binding domain of the disclosureprevents or inhibits the binding of nucleotides, in particular guaninenucleotides or analogs (as described hereinbefore), to the G protein. Oralso, the binding domain of the disclosure is capable of displacing aguanine nucleotide or analog from the G protein. Non-limiting examplesof assays to determine the degree of inhibition of binding ordisplacement of guanine nucleotides to a protein are provided in theExample section, for example in Example 3 and 8. Further, it will beappreciated that all particular embodiments related to the bindingdomains hereof, as described hereinbefore, also apply for thisparticular aspect hereof.

The functional versatility of GPCRs is inherently coupled to theflexibility of these proteins resulting in a spectrum of conformations.The confoimational energy landscape is intrinsically coupled to suchfactors as the presence of bound ligands (effector molecules, agonists,antagonists, inverse agonists, etc.), the lipid environment or thebinding of interacting proteins. Thus, in one embodiment, the bindingdomains hereof, in particular the immunoglobulin single variable domainsincrease the stability of the complex comprising a GPCR and a G proteinupon binding of the binding domain. hi still a further embodiment,binding of the binding domain of the disclosure to any conformationalepitope that is represented by or accessible on or part of a complexcomprising a GPCR:G protein complex, induces the formation of afunctional conformational state of a GPCR, in particular an activeconfoimational state of the GPCR. More specifically, the binding domainof the disclosure is capable of stabilizing the active state of the GPCRcomprised in the GPCR:G protein complex, by increasing the affinity ofthe G protein for the receptor. Likewise, the binding domain of thepresent disclosure is capable of stabilizing an agonist-bound GPCR:Gprotein complex and/or enhances the affinity of an agonist for a GPCR:Gprotein complex. Preferably, the binding domain is capable of increasingthe affinity of the G protein for the GPCR and/or the affinity of theagonist for the GPCR:G protein complex at least twofold, at leastfivefold, and more preferably at least tenfold as measured by a decreasein K_(d). Alternatively, the binding domain is capable of inducing ashift in EC₅₀ or IC₅₀ by at least twofold, at least fivefold, and morepreferably at least tenfold in any assay set-up comparing theinteraction strength of the receptor and the G protein in presence ofthe complex stabilizing binding domain versus a condition where thisbinding domain is absent or any other measure of affinity or potencyknown to one of skill in the art.

The term “a functional conformational state,” as used herein, refers tothe fact that proteins, in particular membrane proteins such as GPCRs,possess many different conformational states having a dynamic range ofactivity, in particular ranging from no activity to maximal activity(reviewed in Kobilka and Deupi, 2007). It should be clear that “afunctional conformational state” is not meant to cover the denaturedstates of proteins. For example, a basal conformational state can bedefined as a low energy state of the receptor in the absence of aligand. The probability that a protein will undergo a transition toanother conformational state is a function of the energy differencebetween the two states and the height of the energy barrier between thetwo states. In the case of a receptor protein, such as a GPCR, theenergy of ligand binding can be used either to alter the energy barrierbetween the two states, or to change the relative energy levels betweenthe two states, or both. Changing of the energy barrier would have aneffect on the rate of transition between the two states, whereaschanging the energy levels would have an effect on the equilibriumdistribution of receptors in two states. Binding of an agonist orpartial agonist would lower the energy barrier and/or reduce the energyof the more active conformational state relative to the inactiveconformational state. An inverse agonist would increase the energybarrier and/or reduce the energy of the inactive state conformationrelative to the active conformation. Coupling of the receptor to its Gprotein could further alter the energy landscape. Cooperativeinteractions of β2AR and Gs observed in ligand binding assays formed thefoundation of the ternary complex model of GPCR activation (Delean etal., 1980). In the ternary complex consisting of agonist, receptor, andG protein, the affinity of the receptor for agonist is enhanced and thespecificity of the G protein for guanine nucleotides changes in favor ofGTP over GDP.

It should be noted that the activities of integral membrane proteins,including GPCRs are also affected by the structures of the lipidmolecules that surround them in the membrane. Membrane proteins are notrigid entities, and deform to ensure good hydrophobic matching to thesurrounding lipid bilayer. One important parameter is the hydrophobicthickness of the lipid bilayer, defined by the lengths of the lipidfatty acyl chains. Also, the structure of the lipid headgroup region islikely to be important in defining the structures of those parts of amembrane protein that are located in the lipid headgroup region. Amongother lipids, palmitoylation and binding of cholesterol to GPCRs mayalso play a structural role inside monomeric receptors and contribute tothe formation/stabilization of receptor oligomers (Lee 2004; Chini andParenti 2009).

A further aspect of the present disclosure relates to a complexcomprising a binding domain hereof. More specifically, a complex isprovided comprising a binding domain hereof, a GPCR, a G protein, andoptionally a receptor ligand. As a non-limiting example, a stablecomplex may be purified by gel filtration, as was done for example forthe quaternary complex containing a nanobody, a GPCR, a G protein, and areceptor ligand (see Example section). In a particular embodiment, thecomplex can be crystalline. Accordingly, a crystal of the complex isalso provided, as well as methods of making the crystal, which aredescribed in more detail further herein.

In another aspect, a nucleic acid sequence encoding an amino acidsequence of any of the binding domains hereof, in particularimmunoglobulin single variable domains, is also part of the presentdisclosure and non-limiting examples are provided in Table 4. Accordingto preferred embodiments, the disclosure relates to nucleic acidsequences of binding domains hereof, in particular immunoglobulin singlevariable domains, in which the sequences have more than 80%, preferablymore than 90%, more preferably more than 95%, such as 99% or moresequence identity (as defined herein) with the sequences of at least oneof the nucleic acid sequences of the binding domains of SEQ ID NO:s49-54 (see Table 4). For the calculation of the percentage sequenceidentity, the nucleic acid sequences of tags (e.g., His tag or EPEA tag)should be disregarded. Also, the nucleic acid sequences as describedherein may be comprised in a nucleic acid sequence.

Further, the present disclosure also envisages expression vectorscomprising nucleic acid sequences encoding any of binding domainshereof, in particular immunoglobulin single variable domains, as well ashost cells expressing such expression vectors. Suitable expressionsystems include constitutive and inducible expression systems inbacteria or yeasts, virus expression systems, such as baculovirus,semliki forest virus and lentiviruses, or transient transfection ininsect or mammalian cells. The cloning, expression and/or purificationof the binding domains hereof, in particular the immunoglobulin singlevariable domains, can be done according to techniques known by theskilled person in the art.

Thus, the present disclosure encompasses a cell or a culture of cellsexpressing a binding domain hereof, in particular an immunoglobulinsingle variable domain that is directed against and/or capable ofspecifically binds to a complex comprising a GPCR and a G protein. Thecells according to the present disclosure can be of any prokaryotic oreukaryotic organism. Preferably, cells are eukaryotic cells, for exampleyeast cells, or insect cells, or cultured cell lines, for examplemammalian cell lines, preferably human cell lines, that endogenously orrecombinantly express a GPCR and/or G protein of interest. The nature ofthe cells used will typically depend on the ease and cost of producingthe native protein(s), the desired glycosylation properties, the originof the target protein, the intended application, or any combinationthereof. Eukaryotic cell or cell lines for protein production are wellknown in the art, including cell lines with modified glycosylationpathways, and non-limiting examples will be provided hereafter.

Animal or mammalian host cells suitable for harboring, expressing, andproducing proteins for subsequent isolation and/or purification includeChinese hamster ovary cells (CHO), such as CHO-K1 (ATCC CCL-61), DG44(Chasin et al., 1986, Som. Cell Molec. Genet., 12:555-556; and Kolkekaret al., 1997, Biochemistry, 36:10901-10909), CHO-K1 Tet-On cell line(Clontech), CHO designated ECACC 85050302 (CAMR, Salisbury, Wiltshire,UK), CHO clone 13 (GEIMG, Genova, IT), CHO clone B (GEIMG, Genova, IT),CHO-K1/SF designated ECACC 93061607 (CAMR, Salisbury, Wiltshire, UK),RR-CHOK1 designated ECACC 92052129 (CAMR, Salisbury, Wiltshire, UK),dihydrofolate reductase negative CHO cells (CHO/-DHFR, Urlaub andChasin, 1980, Proc. Natl. Acad. Sci. USA, 77:4216), and dp12.CHO cells(U.S. Pat. No. 5,721,121); monkey kidney CV1 cells transformed by SV40(COS cells, COS-7, ATCC CRL-1651); human embryonic kidney cells (e.g.,293 cells, or 293T cells, or 293 cells subcloned for growth insuspension culture, Graham et al., 1977, J. Gen. Virol., 36:59, or GnTIKO HEK293S cells, Reeves et al., 2002, PNAS, 99: 13419); baby hamsterkidney cells (BHK, ATCC CCL-10); monkey kidney cells (CV1, ATCC CCL-70);African green monkey kidney cells (VERO-76, ATCC CRL-1587; VERO, ATCCCCL-81); mouse sertoli cells (TM4, Mather, 1980, Biol. Reprod.,23:243-251); human cervical carcinoma cells (HELA, ATCC CCL-2); caninekidney cells (MDCK, ATCC CCL-34); human lung cells (W138, ATCC CCL-75);human hepatoma cells (HEP-G2, HB 8065); mouse mammary tumor cells (MMT060562, ATCC CCL-51); buffalo rat liver cells (BRL 3A, ATCC CRL-1442);TRI cells (Mather, 1982, Annals N.Y. Acad. Sci., 383:44-68); MCR 5cells; FS4 cells. According to a particular embodiment, the cells aremammalian cells selected from Hek293 cells or COS cells.

Exemplary non-mammalian cell lines include, but are not limited to, Sf9cells, baculovirus-insect cell systems (e.g., review Jarvis, VirologyVolume 310, Issue 1, 25 May 2003, Pages 1-7), plant cells such astobacco cells, tomato cells, maize cells, algae cells, or yeasts such asSaccharomyces species, Schizosaccharomyces species, Hansenula species,Yarrowia species or Pichia species. According to particular embodiments,the eukaryotic cells are yeast cells from a Saccharomyces species (e.g.,Saccharomyces cerevisiae), Schizosaccharomyces sp. (for exampleSchizosaccharomyces pombe), a Hansenula species (e.g., Hansenulapolymorpha), a Yarrowia species (e.g., Yarrowia lipolytica), aKluyveromyces species (e.g., Kluyveromyces lactis), a Pichia species(e.g., Pichia pastoris), or a Komagataella species (e.g., Komagataellapastoris). According to a specific embodiment, the eukaryotic cells arePichia cells, and in a most particular embodiment Pichia pastoris cells.

Transfection of target cells (e.g., mammalian cells) can be carried outfollowing principles outlined by Sambrook and Russel (Molecular Cloning,A Laboratory Manual, 3^(rd) Edition, Volume 3, Chapter 16, Section16.1-16.54). In addition, viral transduction can also be performed usingreagents such as adenoviral vectors. Selection of the appropriate viralvector system, regulatory regions and host cell is common knowledgewithin the level of ordinary skill in the art. The resulting transfectedcells are maintained in culture or frozen for later use according tostandard practices.

Accordingly, another aspect of the disclosure relates to a method forproducing a binding domain according to the invention, the methodcomprising at least the steps of:

-   -   a) Expressing in a suitable cellular expression system (as        defined hereinabove) a nucleic acid according to the invention,        and optionally    -   b) Isolating and/or purifying the binding domain.

The herein described binding domains, complexes, cells or cell lines canbe can be used in a variety of contexts and applications, for example,and without limitation, for capturing and/or purification of GPCR:Gprotein complexes, and in crystallization studies and high-resolutionstructural analysis of GPCR:G protein complexes. It is thus one of theaims of the disclosure to use the binding domain according to theinvention, in particular immunoglobulin single variable domains, such asnanobodies, as tools to stabilize GPCR:G protein complexes and furtherto use these binding domains as co-crystallization aids for GPCRs incomplex with a G protein, or in other words to facilitatecrystallogenesis of GPCR:G protein complexes. Additionally, and/oralternatively, the binding domains and preferably cellular systemsexpressing the binding domains, as described herein, can be useful forother applications such as ligand screening, drug discovery,immunization, all of which will be described into further detail below.

Stabilization of a GPCR:G Protein Complex and Locking the GPCR in the GProtein-Bound State

Thus, according to one aspect, the disclosure relates to the use of abinding domain as described hereinbefore to stabilize a complexcomprising a GPCR and a G protein. According to a preferred embodiment,the complex that is stabilized further comprises a receptor ligand, morespecifically an agonist. The term “stabilize,” “stabilizing,” or“increasing the stability,” as used herein, refers to increasing thestability of a GPCR:G protein complex with respect to the structure(conformational state) and/or particular biological activity(intracellular signaling activity) of one or both of the constitutingproteins of the complex, in particular the GPCR and/or the G protein. Inone particularly preferred embodiment, the binding domain of thedisclosure can be used to stabilize the GPCR:G protein complex so thatthe GPCR is locked or fixed in an active or G protein-bound state. AGPCR that adopts such an active or G protein-bound state will exert itsbiological activity in nature. Ways to determine the (increased)stability of a GPCR:G protein complex have been described hereinbeforeand are further illustrated in the Examples section.

It will be appreciated that having increased stability with respect tostructure and/or a particular biological activity of a GPCR includes thestability to other denaturants or denaturing conditions including heat,a detergent, a chaotropic agent and an extreme pH. Accordingly, in afurther embodiment, the binding domain according to the disclosure iscapable of increasing the stability of a GPCR:G protein complex undernon-physiological conditions induced by dilution, concentration, buffercomposition, heating, cooling, freezing, detergent, chaotropic agent,pH, amongst others. Accordingly, the term “thermostabilize,”“thermostabilizing,” “increasing the thermostability of,” refers to thefunctional rather than to the thermodynamic properties of a GPCR:Gprotein complex and to the constituting protein's resistance toirreversible denaturation induced by thermal and/or chemical approachesincluding but not limited to heating, cooling, freezing, chemicaldenaturants, pH, detergents, salts, additives, proteases or temperature.Irreversible denaturation leads to the irreversible unfolding of thefunctional conformations of the protein, loss of biological activity andaggregation of the denaturated protein. The term “(thermo)stabilize,”“(thermo)stabilizing,” “increasing the (thermo)stability of,” as usedherein, applies to GPCR:G protein complexes embedded in lipid particlesor lipid layers (for example, lipid monolayers, lipid bilayers, and thelike) and to GPCR:G protein complexes that have been solubilized indetergent.

In relation to an increased stability to heat, this can be readilydetermined by measuring ligand binding or by using spectroscopic methodssuch as fluorescence, CD or light scattering that are sensitive tounfolding at increasing temperatures. It is preferred that the bindingdomain is capable of increasing the stability as measured by an increasein the thermal stability of a GPCR:G protein complex with at least 2°C., at least 5° C., at least 8° C., and more preferably at least 10° C.or 15° C. or 20° C. According to another preferred embodiment, thebinding domain is capable of increasing the thermal stability of aGPCR:G protein complex with a receptor ligand, more specifically anagonist or positive allosteric modulator of the GPCR dependent signalingpathway. According to another preferred embodiment, the binding domainaccording to the disclosure is capable of increasing the stability of aGPCR:G protein complex in the presence of a detergent or a chaotrope.Preferably, the binding domain is capable of increasing the stability ofa GPCR:G protein complex to denaturation induced by thermal or chemicalapproaches. In relation to an increased stability to heat, a detergentor to a chaotrope, typically the GPCR:G protein is incubated for adefined time in the presence of a test detergent or a test chaotropicagent and the stability is detennined using, for example, ligand bindingor a spectroscoptic method, optionally at increasing temperatures asdiscussed above. According to still another preferred embodiment, thebinding domain according to the disclosure is capable of increasing thestability to extreme pH of a functional conformational state of a GPCR.Preferably, the binding domain is capable of increasing the stability ofa GPCR:G protein complex to extreme pH. In relation to an extreme of pH,a typical test pH would be chosen for example in the range 6 to 8, therange 5.5 to 8.5, the range 5 to 9, the range 4.5 to 9.5, morespecifically in the range 4.5 to 5.5 (low pH) or in the range 8.5 to 9.5(high pH).

In a particularly preferred embodiment, the binding domain according tothe disclosure can be used to prevent the dissociation of the complex inthe presence of nucleotides, in particular guanine nucleotides oranalogs thereof. More specifically, guanine nucleotides include GDP andGTP, and analogs of guanine nucleotides include, without beinglimitative, GTPγS or GDP in combination with aluminum or berylliumfluoride species or nucleotide fragments such as pyrophosphate orfoscarnet.

Capturing and/or Purifying a GPCR:G Protein Complex

It will thus be understood that the ability of forming a stable GPCR:Gprotein complex is particularly useful for capturing and/or purifying aGPCR:G protein complex, which will allow subsequent crystallization,ligand characterization and compound screening, immunizations, amongstothers. Moreover, it is of particular advantage that the binding domainsof the disclosure can be useful generic tools that may be applicable fora range of GPCR:G protein complexes.

Accordingly, the present disclosure also envisages a method of capturingand/or purifying a complex comprising a GPCR and a G protein, the methodcomprising the steps of:

-   -   a) Providing a binding domain according to the invention, and    -   b) Allowing the binding domain to bind to a complex comprising a        GPCR and a G protein, and    -   c) Optionally, isolating the complex foimed in step b).

In a specific embodiment, the disclosure provides for a method ofcapturing a complex comprising a GPCR and a G protein comprising thesteps of:

-   -   a) applying a solution containing a plurality of GPCRs and G        proteins to a solid support possessing an immobilized binding        domain according to the invention, and    -   b) Forming a complex of the binding domain, the GPCR and the G        protein, and    -   c) Removing weakly bound or unbound molecules,

The present disclosure also envisages a method of purifying a complexcomprising a GPCR and a G protein, the method comprising the steps of:

-   -   a) Contacting a solution containing a GPCR and a G protein with        a binding domain according to the invention, and    -   b) Forming a complex comprising the binding domain, the GPCR and        the G protein, and    -   c) Isolating the complex of step b)

wherein a complex of a GPCR and a G protein is essentially purified.

According to a particular embodiment, the binding domain as describedherein can also be used to capture a target GPCR:G protein complexfurther comprising a receptor ligand and/or one or more otherinteracting proteins.

The above methods for capturing/purifying target GPCR:G proteincomplexes include, without limitation, affinity-based methods such asaffinity chromatography, affinity purification, immunoprecipitation,protein detection, immunochemistry, surface-display, amongst others, andare all well-known in the art.

Crystallization and Resolving the Structure of a GPCR:G Protein Complex

Crystallization of membrane proteins including GPCRs remains aformidable challenge. Although expression and purification methods areappearing that allow for the generation of milligram quantities,achieving stability with these molecules is perhaps the most difficulthurdle to overcome. First of all, binding domains according to thedisclosure may increase the stability of detergent solubilized GPCR:Gprotein complexes, protecting them from proteolytic degradation and/oraggregation and facilitating the purification and concentration ofhomogenous samples of correctly folded proteins. Persons of ordinaryskill in the art will recognize that such samples are the preferredstarting point for the generation of diffracting crystals.

Crystallization is another major bottleneck in the process ofmacromolecular structure determination by X-ray crystallography.Successful crystallization requires the formation of nuclei and theirsubsequent growth to crystals of suitable size. Crystal growth generallyoccurs spontaneously in a supersaturated solution as a result ofhomogenous nucleation. Proteins may be crystallized in a typical sparsematrix screening experiment, in which precipitants, additives andprotein concentration are sampled extensively, and supersaturationconditions suitable for nucleation and crystal growth can be identifiedfor a particular protein. Related to the sparse matrix screeningapproach is to generate structural variation in the protein itself, forexample by adding ligands that bind the protein, or by making differentmutations, preferentially in surface residues of the target protein orby trying to crystallize different species orthologues of the targetprotein (Chang 1998). One unexpected finding of the present disclosureis the usefulness of binding domains that specifically bind to a GPCR:Gprotein complex to introduce a degree of structural variation uponbinding while preserving the overall fold of the complex.

Because crystallization involves an unfavorable loss of conformationalentropy in the molecule to be assembled in the crystal lattice, methodsthat reduce the conformational entropy of the target while still insolution should enhance the likelihood of crystallization by loweringthe net entropic penalty of lattice formation. The “surface entropyreduction” approach has proved to be highly effective (Derewenda 2004).Likewise, binding partners such as ions, small molecule ligands, andpeptides can reduce the confoimational heterogeneity by binding to andstabilizing a subset of conformational states of a protein. Althoughsuch binding partners are effective, not all proteins have a knownbinding partner, and even when a binding partner is known, its affinity,solubility, and chemical stability may not be compatible withcrystallization trials. Therefore, it was surprisingly found that thebinding domains of the present disclosure can be used as tools toincrease the probability of obtaining well-ordered crystals byminimizing the conformational heterogeneity in the target GPCR:G proteincomplex by binding to a specific conformation of G protein.

Crystallization of GPCRs for high-resolution structural studies isparticularly difficult because of the amphipathic surface of thesemembrane proteins. Embedded in the membrane bilayer, the contact sitesof the protein with the acyl chains of the phospholipids arehydrophobic, whereas the polar surfaces are exposed to the polar headgroups of the lipids and to the aqueous phases. To obtain well-orderedthree-dimensional crystals—a prerequisite to X-ray structural analysisat high resolution—GPCRs are solubilized with the help of detergents andpurified as protein—detergent complexes. The detergent micelle coversthe hydrophobic surface of the membrane protein in a belt-like manner(Hunte and Michel 2002; Ostermeier et al., 1995). GPCR—detergentcomplexes form three-dimensional crystals in which contacts betweenadjacent protein molecules are made by the polar surfaces of the proteinprotruding from the detergent micelle (Day et al., 2007). Obviously, thedetergent micelle requires space in the crystal lattice. Althoughattractive interactions between the micelles might stabilize the crystalpacking (Rasmussen et al., 2007; Dunn et al., 1997), these interactionsdo not lead to rigid crystal contacts.

Because many membrane proteins, including GPCRs contain relatively smallor highly flexible hydrophilic domains, a strategy to increase theprobability of getting well-ordered crystals is to enlarge the polarsurface of the protein and/or to reduce their flexibility. The mostphysiologic approach is to use a native signaling partner such as a Gprotein or arrestin. Unfortunately, interactions of GPCRs with Gproteins or arrestins are highly lipid dependent, and it has beendifficult to form complexes of sufficient stability for crystallography.So, the binding domains of the present disclosure can be used to enlargethe polar surfaces of the GPCRs through binding of a G protein,supplementing the amount of protein surface that can facilitate primarycontacts between molecules in the crystal lattice with the polarsurfaces of the G protein and the nanobody. Binding domains of thepresent disclosure can also reduce the flexibility of its extracellularregions to grow well-ordered crystals. Immunoglobulin single variabledomains, including nanobodies, are especially suited for this purposebecause they bind conformational epitopes and are composed of one singlerigid globular domain, devoid of flexible linker regions unlikeconventional antibodies or fragments derived such as Fabs.

Thus, according to a preferred embodiment, the present disclosureprovides for binding domains useful as tools to crystallize a complexcomprising a GPCR and a G protein, and eventually to solve thestructure. More preferably, the complex which is crystallized by makingusing of a binding domain of the present disclosure further comprises areceptor ligand, more specifically an agonist. In a particularlypreferred embodiment, the GPCR comprised in the complex is in an activestate or conformation.

Thus, the binding domain in complex with the GPCR:G protein complex andoptionally receptor ligand may be crystallized using any of a variety ofspecialized crystallization methods for membrane proteins, many of whichare reviewed in Caffrey (2003 & 2009). In general terms, the methods arelipid-based methods that include adding lipid to the complex prior tocrystallization. Such methods have previously been used to crystallizeother membrane proteins. Many of these methods, including the lipidiccubic phase crystallization method and the bicelle crystallizationmethod, exploit the spontaneous self-assembling properties of lipids anddetergent as vesicles (vesicle-fusion method), discoidal micelles(bicelle method), and liquid crystals or mesophases (in meso orcubic-phase method). Lipidic cubic phases crystallization methods aredescribed in, for example: Landau et al., 1996; Gouaux 1998; Rummel etal., 1998; Nollert et al., 2004, Rasmussen et al., 2011, whichpublications are incorporated by reference for disclosure of thosemethods. Bicelle crystallization methods are described in, for example:Faham et al., 2005; Faham et al., 2002, which publications areincorporated by reference for disclosure of those methods.

According to another embodiment, the disclosure relates to the use of abinding domain as described herein to solve a structure of a targetcomplex comprising a GPCR and a G protein, and optionally furthercomprising a receptor ligand. “Solving the structure” as used hereinrefers to determining the arrangement of atoms or the atomic coordinatesof a protein, and is often done by a biophysical method, such as X-raycrystallography.

In x-ray crystallography, the diffraction data when properly assembledgives the amplitude of the 3D Fourier transform of the molecule'selectron density in the unit cell. If the phases are known, the electrondensity can be simply obtained by Fourier synthesis. For a proteincomplex, the success to derive phase information from molecularreplacement (MR) alone is questionable when the fraction of proteinswith a known structure (the search models) is low (less than 50% of theamino acid content) and/or when the crystals exhibit limited diffractionquality. While the combination of multiple isomorphous replacement (MIR)and MR phasing has proven successful for protein complexes (e.g.,Ostermeier et al., 1995; Li et al., 1997; Hunte et al., 2000), therequirement of producing a good heavy atom derivative is almost alwaysproblematic. Over the past decade, classical MIR approaches havegenerally been superseded by the use of anomalous dispersion dataprincipally using selenomethionine (SeMet) incorporation (MAD or SAD)(Hendrickson 1991). In fact, the anomalous experimental data usingSe-edge energies generally provide superior and less biased phaseinformation compared with either MIR or model-based MR phasing data.Accordingly, one specific embodiment relates to the use of a bindingdomain according to the disclosure for the phasing of GPCR:G complexesby MR or MAD. In particular, immunoglobulin single variable domains,including nanobodies, generally express robustly and are suitable forSeMet incorporation. To illustrate this further, and without beinglimitative, phasing a complex comprising a GPCR, G protein, and ananobody by introducing all the SeMet sites in the nanobody alonecircumvents the need to incorporate SeMet sites in the GPCR or the Gprotein.

In many cases, obtaining a diffraction-quality crystal is the chiefbarrier to solving its atomic-resolution structure. Thus, according tospecific embodiments, the herein described binding domains can be usedto improve the diffraction quality of the crystals so that the crystalstructure of the target complex can be solved.

Further, obtaining structural information of GPCR targets, for exampleto help guide GPCR drug discovery, is highly desired. Beyond thecrystallization of more GPCRs, especially methods for acquiringstructures of receptors bound to different classes of ligands includingagonists, antagonists, allosteric regulators and/or G proteins areneeded. The present disclosure particularly provides general tools toobtain crystals of GPCR:G protein complexes. In particular,agonist-bound GPCR:G protein complex crystals may providethree-dimensional representations of the active states of GPCRs. Thesestructures will help clarify the conformational changes connecting theligand-binding and G protein-interaction sites, and lead to more precisemechanistic hypotheses and eventually new therapeutics. Given theconformational flexibility inherent to ligand-activated GPCRs and thegreater heterogeneity exhibited by agonist-bound receptors, stabilizingsuch a state is not easy. Thus, such efforts can benefit from thestabilization of a complex of an agonist-bound receptor conformationbound to its heterotrimeric G protein by the addition of binding domainsthat are specific for such a complex. Especially suited are bindingdomains that bind to the G protein that forms part of such a complex,since these binding domains can be used as general tools to stabilizeall GPCRs that signal through the same G protein (e.g., Gs coupledreceptors, Gi coupled receptors, etc.).

According to an alternative embodiment, the present disclosureencompasses a method of determining the crystal structure of a complexcomprising a GPCR and a G protein, the method comprising the steps of:

-   -   a) Providing a binding domain according to the invention, and    -   b) Allowing the binding domain to bind to a complex comprising a        GPCR and a G protein, and    -   c) Crystallizing the complex formed in step b).

In particular embodiments of the above method of determining the crystalstructure, the target complex comprising a GPCR and a G protein furthercomprises a receptor ligand, more specifically an agonist, bound to theGPCR.

The determining of the crystal structure may be done by a biophysicalmethod such as X-ray crystallography. The method may further comprise astep for obtaining the atomic coordinates of the crystal (as definedhereinbefore).

Identification of Compounds Targeting a GPCR:G Protein Complex

In the process of compound screening, drug discovery and leadoptimization, there is a requirement for faster, more effective, lessexpensive and especially information-rich screening assays that providesimultaneous information on various compound characteristics and theiraffects on various cellular pathways (i.e., efficacy, specificity,toxicity and drug metabolism). Thus, there is a need to quickly andinexpensively screen large numbers of compounds in order to identify newspecific ligands of a GPCR of interest, preferably conformation specificligands, which may be potential new drug candidates. The presentdisclosure solves this problem by providing binding domains thatstabilize a GPCR:G protein complex in a functional conformational state,that can then be used as immunogen or selection reagent for screening ina variety of contexts.

A major advantage of the binding domains according to the disclosure isthat the GPCR as comprised in the GPCR:G protein complex can be kept ina stabilized functional conformation, particularly in an active stateconformation. For example, library compounds that selectively bind thisactive conformation of the receptor have a higher propensity to behaveas agonists because orthosteric or allosteric stabilization of theactive conformation of the GPCR elicits biological responses.

Another advantage is that the binding domain increases thethermostability of the active conformation of the GPCR comprised in thecomplex, thus protecting the GPCR against irreversible or thermaldenaturation induced by the non-native conditions used in compoundscreening and drug discovery, without the need to rely on mutant GPCRswith increased stability.

Another major advantage of the conformation-selective binding domainsaccording to the disclosure is that they allow to quickly and reliablyscreen for and differentiate between receptor agonists, inverseagonists, antagonists and/or modulators as well as inhibitors of GPCRsand GPCR-dependent pathways, so increasing the likelihood of identifyinga ligand with the desired pharmacological properties.

To illustrate this further, it is a well-established concept that mostGPCRs exhibit higher agonist binding affinity when complexed with Gprotein. This is attributed to the cooperative interaction betweenagonist occupied receptor and G protein. Binding domains of the presentdisclosure that inhibit the dissociation of a complex of a GPCR and a Gprotein thus will stabilize an active conformational state of the R:Gprotein complex, thus increasing the affinity of the GPCR for agonistsand decreasing the affinity for inverse agonists. It follows thatbinding domains that recognize the active functional conformation of theR:G complex can for example be used in high-throughput screening assaysto screen for agonists because they increase the affinity of thereceptor for agonists, relative to inverse agonists. Binding domainsthat recognize the active functional conformation of the G:R complex canalso be used in high-throughput screening assays to screen for biasedagonists with the ability to selectively stimulate a subset of areceptor's signaling activities, for example the selective activation ofG protein, relative to β-arrestin function.

According to a specific embodiment, a binding domain which specificallybinds to the G protein in complex with a GPCR (e.g., Tables 2 and 3) canbe used as a universal tool for screening programs targeting a pluralityof GPCRs, since a particular G protein (e.g., Gs) will form a complexwith a plurality of GPCRs (e.g., Gs coupled receptors, including 5-HTreceptors types 5-HT₄ and 5-HT₇, ACTH receptor, Adenosine receptor typesA_(2a) and A_(2b), Arginine vasopressin receptor 2, β-adrenergicreceptors types β₁, β₂ and β₃, Calcitonin receptor, Calcitoningene-related peptide receptor, Corticotropin-releasing hormone receptor,Dopamine receptors D₁-like family (D₁ and D₅), FSH-receptor, Gastricinhibitory polypeptide receptor, Glucagon receptor, Histamine H₂receptor, Luteinizing hormone/choriogonadotropin receptor, Melanocortinreceptor, Parathyroid hormone receptor 1, Prostaglandin receptor typesD₂ and I₂, Secretin receptor, Thyrotropin receptor, etc.; see also Table1).

Thus, another aspect according to the present disclosure encompasses theuse of a binding domain, or the use of a complex, a cell, a membranepreparation comprising a binding domain, all as described hereinbefore,in screening and/or identification programs for conformation-specificbinding partners of a GPCR:G protein complex, which ultimately mightlead to potential new drug candidates.

According to one embodiment, the disclosure envisages a method ofidentifying compounds capable of selectively binding to a GPCR:G proteincomplex, the method comprising the steps of:

-   -   (i) Providing a complex comprising a GPCR and a G protein    -   (ii) Contacting the complex with a binding domain that is        directed against and/or specifically binds to the complex and        allowing the binding domain to bind to the complex, and    -   (iii) Providing a test compound, and    -   (iv) Evaluating whether the test compound binds to the complex,        and    -   (v) Selecting a compound that selectively binds to the complex.

It will be clear that the binding domain as used in any of the abovemethods is capable of stabilizing the functional conformational state ofthe GPCR:G protein complex and prevents the dissociation of the complex.Preferably, the GPCR:G protein complex is in an active conformationalstate (as defined hereinbefore). According to particularly preferredembodiments of the above screening methods, the GPCR:G protein complexfurther comprises a receptor ligand.

It should be noted that particularly preferred embodiments of thebinding domains are as described hereinbefore with respect to theearlier aspects of the disclosure.

Thus, the binding domains of the present disclosure can be useful inscreening assays. Screening assays for drug discovery can be solid phaseor solution phase assays, e.g., a binding assay, such as radioligandbinding assays. It will be appreciated that in some instances highthroughput screening of test compounds is preferred and that the methodsas described above may be used as a “library screening” method, a termwell known to those skilled in the art. Thus, the test compound may be alibrary of test compounds. In particular, high-throughput screeningassays for therapeutic compounds such as agonists, antagonists orinverse agonists and/or modulators form part hereof. For high-throughputpurposes, compound libraries may be used such as allosteric compoundlibraries, peptide libraries, antibody libraries, fragment-basedlibraries, synthetic compound libraries, natural compound libraries,phage-display libraries and the like.

Methodologies for preparing and screening such libraries are known inthe art. In one preferred embodiment, high throughput screening methodsinvolve providing a combinatorial chemical or peptide library containinga large number of potential therapeutic ligands. Such “combinatoriallibraries” or “compound libraries” are then screened in one or moreassays, as described herein, to identify those library members(particular chemical species or subclasses) that display a desiredcharacteristic activity. A “compound library” is a collection of storedchemicals usually used ultimately in high-throughput screening A“combinatorial library” is a collection of diverse chemical compoundsgenerated by either chemical synthesis or biological synthesis, bycombining a number of chemical “building blocks” such as reagents.Preparation and screening of combinatorial libraries are well known tothose of skill in the art. The compounds thus identified can serve asconventional “lead compounds” or can themselves be used as potential oractual therapeutics. Thus, in one further embodiment, the screeningmethods as described herein above further comprises modifying a testcompound which has been shown to bind to a conformationally activeGPCR:G protein complex, and determining whether the modified testcompound binds to the GPCR when residing in the particular conformation.

In cases where high-throughput screening of target GPCR:G proteincomplexes for conformation-specific binding partners will be preferred,this will be facilitated by immobilization of the binding domainaccording to the invention, or the GPCR:G protein complex stabilized bythe binding domain, onto a suitable solid surface or support that can bearrayed or otherwise multiplexed. Non-limiting examples of suitablesolid supports include beads, columns, slides, chips or plates. Morespecifically, the solid supports may be particulate (e. g. beads orgranules, generally used in extraction columns) or in sheet form (e. g.membranes or filters, glass or plastic slides, microtiter assay plates,dipstick, capillary fill devices or such like) which can be flat,pleated, or hollow fibers or tubes. The following matrices are given asexamples and are not exhaustive, such examples could include silica(porous amorphous silica), i. e. the FLASH series of cartridgescontaining 60 A irregular silica (32-63 um or 35-70 um) supplied byBiotage (a division of Dyax Corp.), agarose or polyacrylamide supports,for example the Sepharose range of products supplied by AmershamPharmacia Biotech, or the Affi-Gel supports supplied by Bio-Rad. Inaddition there are macroporous polymers, such as the pressure-stableAffi-Prep supports as supplied by Bio-Rad. Other supports that could beutilized include; dextran, collagen, polystyrene, methacrylate, calciumalginate, controlled pore glass, aluminium, titanium and porousceramics. Alternatively, the solid surface may comprise part of a massdependent sensor, for example, a surface plasmon resonance detector.Further examples of commercially available supports are discussed in,for example, Protein Immobilization, R. F. Taylor ed., Marcel Dekker,Inc., New York, (1991).

Immobilization may be either non-covalent or covalent. In particular,non-covalent immobilization or adsorption on a solid surface of thebinding domain, or the GPCR:G protein complex stabilized by the bindingdomain, according to the disclosure may occur via a surface coating withany of an antibody, or streptavidin or avidin, or a metal ion,recognizing a molecular tag attached to the binding domain or the GPCR,according to standard techniques known by the skilled person (e.g.,biotin tag, Histidine tag, etc.). Alternatively, the binding domain, orthe GPCR:G protein complex stabilized by the binding domain, accordingto the invention, may be attached to a solid surface by covalentcross-linking using conventional coupling chemistries. A solid surfacemay naturally comprise cross-linkable residues suitable for covalentattachment or it may be coated or derivatized to introduce suitablecross-linkable groups according to methods well known in the art.

In one particular embodiment, sufficient functionality of theimmobilized protein is retained following direct covalent coupling tothe desired matrix via a reactive moiety that does not contain achemical spacer aim. Further examples and more detailed information onimmobilization methods of antibody (fragments) on solid supports arediscussed in Jung et al. (2008); similarly, membrane receptorimmobilization methods are reviewed in Cooper (2004); both hereinincorporated by reference. Noteably, the mutation of a particular aminoacid (in a protein with known or inferred structure) to a lysine orcysteine (or other desired amino acid) can provide a specific site forcovalent coupling, for example. It is also possible to reengineer aspecific protein to alter the distribution of surface available aminoacids involved in the chemical coupling (Kallwass et al., 1993), ineffect controlling the orientation of the coupled protein. A similarapproach can be applied to the binding domains according to theinvention, as well as to the conformationally stabilized GPCR:G proteincomplexes, so providing a means of oriented immobilization without theaddition of other peptide tails or domains containing either natural orunnatural amino acids. In case of an antibody or an antibody fragment,such as a nanobody, introduction of mutations in the framework region ispreferred, minimizing disruption to the antigen-binding activity of theantibody (fragment).

Conveniently, the immobilized proteins may be used in immunoadsorptionprocesses such as immunoassays, for example ELISA, or immunoaffinitypurification processes by contacting the immobilized proteins accordingto the disclosure with a test sample (i.e., comprising the testcompound, amongst other) according to standard methods conventional inthe art. Alternatively, and particularly for high-throughput purposes,the immobilized proteins can be arrayed or otherwise multiplexed.Preferably, the immobilized proteins according to the disclosure areused for the screening and selection of compounds that specifically bindto a conformationally stabilized GPCR:G protein complex, wherein inparticular the GPCR is in an active conformational state.

It will be appreciated that either the binding domain, or the(conformationally stabilized) GPCR:G protein complex, or itsconstituting proteins may be immobilized, depending on the type ofapplication or the type of screening that needs to be done. Also, thechoice of the GPCR:G protein stabilizing binding domain (targeting aparticular conformational epitope of the GPCR:G protein complex), willdetermine the orientation of the proteins and accordingly, the desiredoutcome of the compound identification, e.g., compounds specificallybinding to extracellular parts, intramembranal parts or intracellularparts of the conformationally stabilized GPCR or compounds specificallybinding to the conformationally stabilized G protein.

Alternatively, the test compound (or a library of test compounds) may beimmobilized on a solid surface, such as a chip surface, whereas thebinding domain and the GPCR:G protein complex are provided, for example,in a detergent solution or in a membrane-like preparation (see below).

Most preferably, neither the binding domain, nor the GPCR:G proteincomplex or its constituting proteins, nor the test compound areimmobilized, as is the case for example in phage-display selectionprotocols in solution, or radioligand binding assays. In a preferredembodiment, the binding domain, the GPCR:G protein complex (orseparately, the constituting proteins) as used in any of the abovescreening methods, are provided as whole cells, or cell (organelle)extracts such as membrane extracts or fractions thereof, or may beincorporated in lipid layers or vesicles (comprising natural and/orsynthetic lipids), high-density lipoparticles, or any nanoparticle, suchas nanodisks, or are provided as VLPs, so that sufficient functionalityof the respective proteins is retained. Preparations of GPCRs formedfrom membrane fragments or membrane-detergent extracts are reviewed indetail in Cooper (2004), incorporated herein by reference.Alternatively, binding domains, GPCR:G protein complexes, or theconstituting proteins may also be solubilized in detergents.Non-limiting examples of solubilized receptor preparations are furtherprovided in the Example section.

Various methods may be used to determine binding between the stabilizedGPCR:G protein complex and a test compound, including for example,enzyme linked immunosorbent assays (ELISA), surface Plasmon resonanceassays, chip-based assays, immunocytofluorescence, yeast two-hybridtechnology and phage display which are common practice in the art, forexample, in Sambrook et al. (2001), Molecular Cloning, A LaboratoryManual. Third Edition. Cold Spring Harbor Laboratory Press, Cold SpringHarbor, N.Y. Other methods of detecting binding between a test compoundand a GPCR include ultrafiltration with ion spray mass spectroscopy/HPLCmethods or other (bio)physical and analytical methods. FluorescenceEnergy Resonance Transfer (FRET) methods, for example, well known tothose skilled in the art, may also be used. It will be appreciated thata bound test compound can be detected using a unique label or tagassociated with the compound, such as a peptide label, a nucleic acidlabel, a chemical label, a fluorescent label, or a radio frequency tag,as described further herein.

The efficacy of the compounds and/or compositions comprising the same,can be tested using any suitable in vitro assay, cell-based assay, invivo assay and/or animal model known per se, or any combination thereof,depending on the specific disease or disorder involved.

In one particular embodiment, it is determined whether the compoundalters the binding of the GPCR to a receptor ligand (as defined herein).Binding of a GPCR to its ligand can be assayed using standard ligandbinding methods known in the art as described herein. For example, aligand may be radiolabeled or fluorescently labeled. The assay may becarried out on whole cells or on membranes obtained from the cells oraqueous solubilized receptor with a detergent. The compound will becharacterized by its ability to alter the binding of the labeled ligand(see also Example section). The compound may decrease the bindingbetween the GPCR and its ligand, or may increase the binding between theGPCR and its ligand, for example by a factor of at least 2 fold, 3 fold,4 fold, 5 fold, 10 fold, 20 fold, 30 fold, 50 fold, 100 fold. Thus,according to more specific embodiments, the complex as used in any ofthe above screening methods further comprises a receptor ligand.Preferably, the receptor ligand is chosen from the group comprising asmall molecule, a polypeptide, an antibody or any fragment derivedthereof, a natural product, and the like. More preferably, the receptorligand is a full agonist, or a partial agonist, or an inverse agonist,or an antagonist, as described hereinbefore.

In addition to establishing binding to a target GPCR:G protein complexin a functional conformational state, it will also be desirable todetermine the functional effect of a compound on the GPCR:G proteincomplex, in particular on the biological activity of the GPCR anddownstream interacting partners. In particular, the binding domainsaccording to the disclosure can be used to screen for compounds thatmodulate (increase or decrease) the biological activity of the GPCR:Gprotein complex, or its constituents, being the GPCR or the G protein.The desired modulation in biological activity will depend on the GPCR ofchoice. The compounds may bind to the target GPCR:G protein complex, inparticular to one or both of its constituents, resulting in themodulation (activation or inhibition) of downstream receptor signaling.This modulation of GPCR signaling can occur ortho- or allosterically.The compounds may bind to the target complex comprising a GPCR bound toa G protein or its constituents so as to activate or increase receptorsignaling; or alternatively so as to decrease or inhibit receptorsignaling. The compounds may also bind to the target complex in such away that they block off the constitutive activity of the GPCR. Thecompounds may also bind to the target complex in such a way that theymediate allosteric modulation (e.g., bind to the GPCR or G protein at anallosteric site). In this way, the compounds may modulate the receptorfunction by binding to different regions in the GPCR;G protein complex(e.g., at allosteric sites). Reference is for example made to George etal. (2002), Kenakin (2002) and Rios et al. (2001). The compounds of thedisclosure may also bind to the target complex in such a way that theyprolong the duration of the GPCR-mediated signaling or that they enhancereceptor signaling by increasing receptor-ligand affinity. Further, thecompounds may also bind to the target complex in such a way that theyinhibit or enhance the assembly of GPCR functional homomers orheteromers.

Also, cell-based assays are critical for assessing the mechanism ofaction of new biological targets and biological activity of chemicalcompounds. Current cell-based assays for GPCRs include measures ofpathway activation (Ca²⁺ release, cAMP generation or transcriptionalactivity); measurements of protein trafficking by tagging GPCRs anddownstream elements with GFP; and direct measures of interactionsbetween proteins using Fórster resonance energy transfer (FRET),bioluminescence resonance energy transfer (BRET) or yeast two-hybridapproaches. Introducing the binding domains of the present invention,inside the cell to the relevant compartment of the cell (intra- orextracellularly) by any means well known and commonly used in the art,may lead to new or better cell-based assays.

In particular, there is a need to “de-orphanize” those GPCRs for which anatural activating ligand has not been identified. The stabilization ofGPCRs in a functional conformational state using the binding domainsaccording to the disclosure enables screening approaches that may beused to identify ligands of “orphan” GPCRs where the natural ligand isunknown. For example, various approaches to “de-orphanization” have beenadopted including array-screening against families of known ligands.Ligands of orphan GPCRs may be identified from biological samples. Thus,in a particular embodiment, the test compound is provided as abiological sample. In particular, the sample can be any suitable sampletaken from an individual. For example, the sample may be a body fluidsample such as blood, serum, plasma, spinal fluid. Alternatively, thesample is tissue or cell extract.

The test compound as used in any of the above screening methods may beselected from the group comprising a polypeptide, a peptide, a smallmolecule, a natural product, a peptidomimetic, a nucleic acid, a lipid,lipopeptide, a carbohydrate, an antibody or any fragment derivedthereof, such as Fab, Fab′ and F(ab′)2, Fd, single-chain Fvs (scFv),single-chain antibodies, disulfide-linked Fvs (dsFv) and fragmentscomprising either a VL or VH domain, a heavy chain antibody (hcAb), asingle domain antibody (sdAb), a minibody, the variable domain derivedfrom camelid heavy chain antibodies (VHH or nanobody), the variabledomain of the new antigen receptors derived from shark antibodies(VNAR), a protein scaffold including an alphabody, protein A, protein G,designed ankyrin-repeat domains (DARPins), fibronectin type III repeats,anticalins, knottins, engineered CH2 domains (nanoantibodies), asdefined hereinbefore.

The test compound may optionally be covalently or non-covalently linkedto a detectable label. Suitable detectable labels and techniques forattaching, using and detecting them will be clear to the skilled person,and include, but are not limited to, any composition detectable byspectroscopic, photochemical, biochemical, immunochemical, electrical,optical or chemical means. Useful labels include magnetic beads (e.g.,dynabeads), fluorescent dyes (e.g., all Alexa Fluor dyes, fluoresceinisothiocyanate, Texas red, rhodamine, green fluorescent protein and thelike), radiolabels (e.g., ³H, ¹²⁵I, ³⁵S, ¹⁴C or ³²P), enzymes (e.g.,horse radish peroxidase, alkaline phosphatase), and colorimetric labelssuch as colloidal gold or colored glass or plastic (e.g., polystyrene,polypropylene, latex, etc.) beads. Means of detecting such labels arewell known to those of skill in the art. Thus, for example, radiolabelsmay be detected using photographic film or scintillation counters,fluorescent markers may be detected using a photodetector to detectemitted illumination. Enzymatic labels are typically detected byproviding the enzyme with a substrate and detecting the reaction productproduced by the action of the enzyme on the substrate, and colorimetriclabels are detected by simply visualizing the colored label. Othersuitable detectable labels were described earlier within the context ofthe first aspect of the disclosure relating to a binding domain.

According to a specifically preferred embodiment, the test compound isan antibody or any fragment derived thereof, as described above,including a nanobody. For example, and without the purpose of beinglimitative, test compounds may be antibodies (as defined herein in itsbroadest sense) that have been raised against a GPCR:G protein complexstabilized by a binding domain according to the invention. Methods forraising antibodies in vivo are known in the art. Preferably,immunization of an animal will be done in a similar way as describedherein before. The disclosure also relates to methods for selectingantibodies specifically binding to a conformationally stabilized GPCR:Gprotein complex, involving the screening of expression librariesencoding immunoglobulin genes, or portions thereof, expressed inbacteria, yeast, filamentous phages, ribosomes or ribosomal subunits orother display systems on the GPCR:G protein complex.

A particular aspect of the present disclosure relates to a solid supportto which is immobilized a binding domain according to the invention.Such a solid support (as described hereinbefore) may thus be used in anyof the above screening methods.

Modulating GPCR Receptor Signaling

The binding domains of the present disclosure can also be used tomodulate GPCR signaling, in particular G protein-mediated GPCRsignaling, including abolishing G protein-mediated GPCR signaling. Theterms “modulating,” “modulation,” and “modulated” means an increase ordecrease in activity of a protein or a protein complex, in particular aGPCR:G protein complex. In particular, the binding domains of thepresent disclosure can be allosteric modulators or allostericinhibitors. The terms “allosteric modulator” or “allosteric inhibitor”in the context of the present disclosure refer to noncompetitivemodulators or inhibitors, which exert their effect by binding to a siteother than the active site of the receptor, and modulate the activity ofthe receptor or render the receptor ineffective in terms of signaltransduction. A “positive allosteric modulator (PAM)” increases signaltransduction, whereas a “negative allosteric modulator (NAM)” reducessignal transduction. In particular, an allosteric inhibitor may alsoabolish signal transduction. Assays to evaluate the modulation in GPCRsignaling by the binding domains of the disclosure are as describedhereinbefore.

In that regard, according to a specific embodiment, the binding domainsof the present invention, in particular immunoglobulin single variabledomains, can also be useful for lead identification and the design ofpeptidomimetics. Using a biologically relevant peptide or proteinstructure as a starting point for lead identification represents one ofthe most powerful approaches in modern drug discovery. Peptidomimeticsare compounds whose essential elements (pharmacophore) mimic a naturalpeptide or protein in 3D space and which retain the ability to interactwith the biological target and produce the same biological effect.Peptidomimetics are designed to circumvent some of the problemsassociated with a natural peptide: for example stability againstproteolysis (duration of activity) and poor bioavailability. Certainother properties, such as receptor selectivity or potency, often can besubstantially improved.

Therapeutic and Diagnostic Applications

Certain of the above-described binding domains may have therapeuticutility and may be administered to a subject having a condition in orderto treat the subject for the condition. The therapeutic utility for abinding domain is determined by the target GPCR:G protein complex towhich the binding domain binds in that signaling via that GPCR is linkedto the condition. hi certain cases, the GPCR may be activated in thecondition by binding to a ligand. In other embodiments, the GPCR may bemutated to make it constitutively active, for example. A subject bindingdomain may be employed for the treatment of a GPCR-mediated conditionsuch as schizophrenia, migraine headache, reflux, asthma, bronchospasm,prostatic hypertrophy, ulcers, epilepsy, angina, allergy, rhinitis,cancer, e.g., prostate cancer, glaucoma and stroke. Further exemplaryGPCR-related conditions at the On-line Mendelian Inheritance in Mandatabase found at the world wide website of the NCBI. So, a particularembodiment of the present disclosure also envisages the binding domainhereof, or a pharmaceutical composition comprising the binding domain,for use in the treatment of a GPCR-related disease or disorder. It willbe appreciated that the therapeutic utility will also depend on theparticular conformational epitope of the GPCR:G protein complex againstwhich the binding domain is directed to.

A subject binding domain may be mixed with another drug substance in afixed pharmaceutical composition or it may be administered separately,before, simultaneously with or after the other drug substance. Ingeneral terms, these protocols involve administering to an individualsuffering from a GPCR-related disease or disorder an effective amount ofa binding domain that modulates the signaling activity of a GPCR in thehost and treat the individual for the disorder.

In some embodiments, where a reduction in activity of a certain GPCR isdesired, one or more compounds that decrease the activity of the GPCRmay be administered, whereas when an increase in activity of a certainGPCR is desired, one or more compounds that increase the activity of theGPCR activity may be administered.

A variety of individuals are treatable according to the subject methods.Generally such individuals are mammals or mammalian, where these termsare used broadly to describe organisms which are within the classmammalia, including the orders carnivore (e.g., dogs and cats), rodentia(e.g., mice, guinea pigs, and rats), and primates (e.g., humans,chimpanzees, and monkeys). In many embodiments, the individuals will behumans. Subject treatment methods are typically performed on individualswith such disorders or on individuals with a desire to avoid suchdisorders.

In another aspect, the disclosure also relates to a pharmaceuticalcomposition comprising a therapeutically effective amount of the bindingdomains of the disclosure and at least one phaii iaceutically acceptablecarrier, adjuvant or diluent.

A “carrier” or “adjuvant,” in particular, a “pharmaceutically acceptablecarrier” or “phaiinaceutically acceptable adjuvant,” is any suitableexcipient, diluent, carrier and/or adjuvant which, by themselves, do notinduce the production of antibodies haiinful to the individual receivingthe composition nor do they elicit protection. So, pharmaceuticallyacceptable carriers are inherently non-toxic and nontherapeutic, andthey are known to the person skilled in the art. Suitable carriers oradjuvantia typically comprise one or more of the compounds included inthe following non-exhaustive list: large slowly metabolizedmacromolecules such as proteins, polysaccharides, polylactic acids,polyglycolic acids, polymeric amino acids, amino acid copolymers andinactive virus particles. Carriers or adjuvants may be, as anon-limiting example, Ringer's solution, dextrose solution or Hank'ssolution. Non aqueous solutions such as fixed oils and ethyl oleate mayalso be used. A preferred excipient is 5% dextrose in saline. Theexcipient may contain minor amounts of additives such as substances thatenhance isotonicity and chemical stability, including buffers andpreservatives.

The administration of a binding domain as described herein or apharmaceutically acceptable salt thereof may be by way of oral, inhaledor parenteral administration. In particular embodiments the nanobody isdelivered through intrathecal or intracerebroventricular administration.The active compound may be administered alone or preferably formulatedas a pharmaceutical composition. An amount effective to treat a certaindisease or disorder that express the antigen recognized by the bindingdomain depends on the usual factors such as the nature and severity ofthe disorder being treated and the weight of the mammal. However, a unitdose will normally be in the range of 0.01 to 50 mg, for example 0.01 to10 mg, or 0.05 to 2 mg of binding domain or a pharmaceuticallyacceptable salt thereof. Unit doses will normally be administered onceor more than once a day, for example 2, 3, or 4 times a day, moreusually 1 to 3 times a day, such that the total daily dose is normallyin the range of 0.0001 to 1 mg/kg; thus a suitable total daily dose fora 70 kg adult is 0.01 to 50 mg, for example 0.01 to 10 mg or moreusually 0.05 to 10 mg. It is greatly preferred that the compound or apharmaceutically acceptable salt thereof is administered in the form ofa unit-dose composition, such as a unit dose oral, parenteral, orinhaled composition. Such compositions are prepared by admixture and aresuitably adapted for oral, inhaled or parenteral administration, and assuch may be in the form of tablets, capsules, oral liquid preparations,powders, granules, lozenges, reconstitutable powders, injectable andinfusable solutions or suspensions or suppositories or aerosols.

Tablets and capsules for oral administration are usually presented in aunit dose, and contain conventional excipients such as binding agents,fillers, diluents, tabletting agents, lubricants, disintegrants,colorants, flavorings, and wetting agents. The tablets may be coatedaccording to well-known methods in the art. Suitable fillers for useinclude cellulose, mannitol, lactose and other similar agents. Suitabledisintegrants include starch, polyvinylpyrrolidone and starchderivatives such as sodium starch glycollate. Suitable lubricantsinclude, for example, magnesium stearate. Suitable pharmaceuticallyacceptable wetting agents include sodium lauryl sulphate. These solidoral compositions may be prepared by conventional methods of blending,filling, tabletting or the like. Repeated blending operations may beused to distribute the active agent throughout those compositionsemploying large quantities of fillers. Such operations are, of course,conventional in the art.

Oral liquid preparations may be in the form of, for example, aqueous oroily suspensions, solutions, emulsions, syrups, or elixirs, or may bepresented as a dry product for reconstitution with water or othersuitable vehicle before use. Such liquid preparations may containconventional additives such as suspending agents, for example sorbitol,syrup, methyl cellulose, gelatin, hydroxyethylcellulose, carboxymethylcellulose, aluminium stearate gel or hydrogenated edible fats,emulsifying agents, for example lecithin, sorbitan monooleate, oracacia; non-aqueous vehicles (which may include edible oils), forexample, almond oil, fractionated coconut oil, oily esters such asesters of glycerine, propylene glycol, or ethyl alcohol; preservatives,for example methyl or propyl p-hydroxybenzoate or sorbic acid, and ifdesired conventional flavoring or coloring agents. Oral formulationsalso include conventional sustained release formulations, such astablets or granules having an enteric coating.

Preferably, compositions for inhalation are presented for administrationto the respiratory tract as a snuff or an aerosol or solution for anebulizer, or as a microfine powder for insufflation, alone or incombination with an inert carrier such as lactose. In such a case theparticles of active compound suitably have diameters of less than 50microns, preferably less than 10 microns, for example between 1 and 5microns, such as between 2 and 5 microns. A favored inhaled dose will bein the range of 0.05 to 2 mg, for example 0.05 to 0.5 mg, 0.1 to 1 mg or0.5 to 2 mg.

For parenteral administration, fluid unit dose forms are preparedcontaining a compound of the present disclosure and a sterile vehicle.The active compound, depending on the vehicle and the concentration, canbe either suspended or dissolved. Parenteral solutions are normallyprepared by dissolving the compound in a vehicle and filter sterilizingbefore filling into a suitable vial or ampoule and sealing.Advantageously, adjuvants such as a local anesthetic, preservatives andbuffering agents are also dissolved in the vehicle. To enhance thestability, the composition can be frozen after filling into the vial andthe water removed under vacuum. Parenteral suspensions are prepared insubstantially the same manner except that the compound is suspended inthe vehicle instead of being dissolved and sterilized by exposure toethylene oxide before suspending in the sterile vehicle. Advantageously,a surfactant or wetting agent is included in the composition tofacilitate uniform distribution of the active compound. Whereappropriate, small amounts of bronchodilators for examplesympathomimetic amines such as isoprenaline, isoetharine, salbutamol,phenylephrine and ephedrine; xanthine derivatives such as theophyllineand aminophylline and corticosteroids such as prednisolone and adrenalstimulants such as ACTH may be included. As is common practice, thecompositions will usually be accompanied by written or printeddirections for use in the medical treatment concerned.

Delivery of binding domains, in particular immunoglobulin singlevariable domains, into cells may be performed as described for peptides,polypeptides and proteins. If the antigen is extracellular or anextracellular domain, the binding domain may exert its function bybinding to this domain, without need for intracellular delivery. Thebinding domains of the present disclosure as described herein may targetintracellular conformational epitopes of GPCR:G proteins of interest. Touse these binding domains as effective and safe therapeutics inside acell, intracellular delivery may be enhanced by protein transduction ordelivery systems know in the art. Protein transduction domains (PTDs)have attracted considerable interest in the drug delivery field fortheir ability to translocate across biological membranes. The PTDs arerelatively short (11-35 amino acid) sequences that confer this apparenttranslocation activity to proteins and other macromolecular cargo towhich they are conjugated, complexed or fused (Sawant and Torchilin2010). The HIV-derived TAT peptide (YGRKKRRQRRR), for example, has beenused widely for intracellular delivery of various agents ranging fromsmall molecules to proteins, peptides, range of pharmaceuticalnanocarriers and imaging agents. Alternatively, receptor-mediatedendocytic mechanisms can also be used for intracellular drug delivery.For example, the transferrin receptor-mediated internalization pathwayis an efficient cellular uptake pathway that has been exploited forsite-specific delivery of drugs and proteins (Qian et al., 2002). Thisis achieved either chemically by conjugation of transferrin withtherapeutic drugs or proteins or genetically by infusion of therapeuticpeptides or proteins into the structure of transferrin. Naturallyexisting proteins (such as the iron-binding protein transferrin) arevery useful in this area of drug targeting since these proteins arebiodegradable, nontoxic, and non-immunogenic. Moreover, they can achievesite-specific targeting due to the high amounts of their receptorspresent on the cell surface. Still other delivery systems include,without the purpose of being limitative, polymer- and liposome-baseddelivery systems.

The efficacy of the binding domains hereof, and of compositionscomprising the same, can be tested using any suitable in vitro assay,cell-based assay, in vivo assay and/or animal model known per se, or anycombination thereof, depending on the specific disease or disorderinvolved.

Screening, Selection, Production of Binding Domains

In still another aspect, the disclosure also encompasses a method ofscreening for binding domains directed against and/or specificallybinding to a complex comprising a GPCR and a G protein, comprising thesteps of:

-   -   a) Providing a plurality of binding domains, and    -   b) Screening the plurality of binding domains for a binding        domain that binds to a complex comprising a GPCR and a G        protein, and    -   c) Isolating the binding domain that binds to the complex.

In a preferred embodiment of this aspect hereof, binding domains aregenerated and screened for their specific binding to a complexcomprising a GPCR and a G protein and optionally a receptor ligand. Thebinding domains may also be generated and screened for their specificbinding to a G protein. As described herein, binding domains can begenerated in many ways. In the case of immunoglobulin single variabledomains, such as nanobodies, typically, immunization of an animal willbe done with a target complex comprising a GPCR bound to a G protein anda receptor ligand, as described hereinbefore (e.g., for V_(H)Hsequences, as a non-limiting example) and also exemplified furtherherein.

For the immunization of an animal with a target complex, the proteins ofthe target complex (i.e., GPCR and G protein) may be produced andpurified using conventional methods that may employ expressing arecombinant form of the proteins in a host cell, and purifying theproteins using affinity chromatography and/or antibody-based methods. Inparticular embodiments, the bactulovirus/Sf-9 system may be employed forexpression, although other expression systems (e.g., bacterial, yeast ormammalian cell systems) may also be used. Exemplary methods forexpressing and purifying GCPRs are described in, for example, Kobilka(1995), Eroglu et al. (2002), Chelikani et al. (2006) and the book“Identification and Expression of G Protein-Coupled Receptors” (Kevin R.Lynch (Ed.), 1998), among many others. A functional GPCR:G proteincomplex may also be reconstituted by using purified receptor (e.g.,β2-AR or MOR) reconstituted into recombinant HDL particles with astoichiometric excess of the G protein (Gs or Gi) as described inWhorton et al. (2009) for β2-AR:Gs or in Kuszak et al. (2009) forMOR:Gi. A GPCR may also be reconstituted in phospholipid vesicles andloaded with a stoichiometric excess of the G protein. Likewise, methodsfor reconstituting an active GPCR in phospholipid vesicles are known,and are described in: Luca et al. (2003), Mansoor et al. (2006), Niu etal. (2005), Shimada et al. (2002), and Eroglu et al. (2003), amongothers. In certain cases, the GPCR and phospholipids may bereconstituted at high density (e.g., 1 mg receptor per mg ofphospholipid). In many cases, a GPCR may be present in the phospholipidvesicle in both orientations (in the normal orientation, and in the“upside down” orientation in which the intracellular loops are on theoutside of the vesicle). Other immunization methods with a GPCR include,without limitation, the use of complete cells expressing a GPCR and/or aG protein or membranes derived thereof.

In a particular embodiment, the animal is immunized with a targetcomplex that is cross-linked with a bifunctional cross-linker (see alsoExample section). Chemical crosslinking can be done using standardtechniques that are well-known by the skilled person in the art (see,e.g., Heimanson, G. T. (2008) Bioconjugate Techniques, 2nd ed., ElsevierInc., 1202 pages).

Any suitable animal, e.g., a warm-blooded animal, in particular a mammalsuch as a rabbit, mouse, rat, camel, sheep, cow or pig or a bird such asa chicken or turkey, may be immunized using any of the techniques wellknown in the art suitable for generating an immune response.

The screening for binding domains specifically binding to aconformational epitope of a target complex may for example be performedby screening a set, collection or library of cells that express thebinding domains on their surface (e.g., B-cells obtained from a suitablyimmunized Camelid), by screening of a (naive or immune) library ofbinding domains, or by screening of a (naive or immune) library ofnucleic acid sequences that encode amino acid sequences of the bindingdomains, which may all be performed in a manner known per se, and whichmethod may optionally further comprise one or more other suitable steps,such as, for example and without limitation, a step of affinitymaturation, a step of expressing the desired amino acid sequence, a stepof screening for binding and/or for activity against the desiredantigen, a step of determining the desired amino acid sequence ornucleotide sequence, a step of introducing one or more humanizingsubstitutions, a step of formatting in a suitable multivalent and/ormultispecific format, a step of screening for the desired biologicaland/or physiological properties (i.e., using a suitable assay known inthe art), and/or any combination of one or more of such steps, in anysuitable order.

Yet another aspect of the disclosure relates to a kit comprising abinding domain according to the invention. The kit may further comprisea combination of reagents such as buffers, molecular tags, vectorconstructs, reference sample material, as well as a suitable solidsupports, cells, nucleic acids, and the like. Such a kit may be usefulfor any of the applications of the present disclosure as describedherein. For example, the kit may comprise (a library of) test compoundsuseful for compound screening applications.

Finally, a last aspect of the disclosure is the use of any bindingdomain according to the disclosure to isolate amino acid sequences thatare responsible for specific binding to a conformational epitope of aGPCR:G protein complex and to construct artificial binding domains basedon the amino acid sequences. It will be appreciated that in the bindingdomains according to the invention, the framework regions and thecomplementarity-determining regions are known, and the study ofderivatives of the binding domain, binding to the same conformationalepitope of a GPCR:G protein complex, will allow deducing the essentialamino acids involved in binding the conformational epitope. Thisknowledge can be used to construct a minimal binding domain and tocreate derivatives thereof, which can routinely be done by techniquesknown by the skilled in the art.

The following examples are intended to promote a further understandingof the present invention. While the present disclosure is describedherein with reference to illustrated embodiments, it should beunderstood that the disclosure is not limited hereto. Those havingordinary skill in the art and access to the teachings herein willrecognize additional modifications and embodiments within the scopethereof. Therefore, the present disclosure is limited only by the claimsattached herein.

EXAMPLES Example 1 Formation and Purification of a StableAgonist-β2AR-Gs Ternary Complex

Formation of a stable complex (see FIG. 2) was accomplished by mixing Gsheterotrimer at approximately 100 μM concentration with BI-167107 boundT4L-β₂AR (or β₂AR-365) in molar excess (approximately 130 μM) in 2 mlbuffer (10 mM HEPES, pH 7.5, 100 mM NaCl, 0.1% DDM, 1 mM EDTA, 3 mMMgCl₂, 10 μM BI-167107) and incubating for 3 hours at room temperature.BI-167107, which was identified from screening and characterizingapproximately 50 different β₂AR agonists, has a dissociation half-timeof approximately 30 hours providing higher degree of stabilization tothe active G protein-bound receptor than other full agonists such asisoproterenol (Rasmussen et al., 2011). To maintain the high-affinitynucleotide-free state of the complex, apyrase (25 mU/ml, NEB) was addedafter 90 min to hydrolyze residual GDP released from Gαs upon binding tothe receptor. GMP resulting from hydrolysis of GDP by apyrase has verypoor affinity for the G protein in the complex. Rebinding of GDP cancause dissociation of the R:G complex (FIG. 3A).

The R:G complex in DDM shows significant dissociation after 48 hours at4° C. (FIG. 4, Panel a). We screened and characterized over 50amphiphiles (data not shown) and identified MNG-3 (NG-310,Affymetrix-Anatrace; Chae et al., 2011) and its closely related analogsas detergents that substantially stabilize the complex (FIG. 4, Panels aand b). The complex was exchanged into MNG-3 by adding the R:G mixture(2 ml) to 8 ml buffer (20 mM HEPES, pH 7.5, 100 mM NaCl, 10 μMBI-167107) containing 1% MNG-3 for 1 hour at room temperature.

At this stage the mixture contains the R:G complex, non-functional Gs,and an excess of β₂AR. To separate functional R:G complex fromnon-functional Gs, and to complete the detergent exchange, the R:Gcomplex was immobilized on M1 Flag resin and washed in buffer (20 mMHEPES, pH 7.5, 100 mM NaCl, 10 μM BI-167107, and 3 mM CaCl₂) containing0.2% MNG-3. To prevent cysteine bridge-mediated aggregation of R:Gcomplexes, 100 μM TCEP was added to the eluted protein prior toconcentrating it with a 50 kDa MWCO Millipore concentrator. The finalsize exclusion chromatography procedure to separate excess free receptorfrom the R:G complex (FIG. 5, Panel b) was performed on a Superdex 20010/300 GL column (GE Healthcare) equilibrated with buffer containing0.02% MNG-3, 10 mM HEPES pH 7.5, 100 mM NaCl, 10 μM BI-167107, and 100μM TCEP. Peak fractions were pooled (FIG. 5, Panel b) and concentratedto approximately 90 mg ml⁻¹ with a 100 kDa MWCO Viva-spin concentratorand analyzed by SDS-PAGE/Coomassie brilliant blue staining (FIG. 5,Panel a) and gel filtration (FIG. 5, Panel c). To confirm a pure,homogeneous, and dephosphorylated preparation, the R:G complex wasroutinely analyzed by ion exchange chromatography (FIG. 5, Panel d).

Example 2 Generation of Nanobodies Binding to the Agonist:β2AR:GsTernary Complex

From negative stain EM imaging (data not shown), we observed that thealpha helical domain of Gαs was flexible. Targeted stabilization of thisdomain was addressed by generating nanobodies that bind to theagonist-β2AR-Gs ternary complex. Nanobodies are single domainantibodies, derived from heavy chain only antibodies from llamas(Muyldermans, 2001). To identify Nanobodies that bind the (agonistloaded) receptor coupled Gs-protein, we immunized two llamas (Llamaglama) with the bis(sulfosuccinimidyl)glutarate (BS2G, Pierce)cross-linked β2AR:Gs:BI167107 ternary complex. Both animals wereimmunized with 4 bi-weekly shots of 50 to 100 μg. After completing theimmunization, peripheral blood lymphocytes were isolated from theimmunized animals to extract total RNA and prepare cDNA. Total RNA wasisolated from about 107 lymphocytes as described by Chomczynski andSacchi (1987). First strand cDNA synthesis was prepared using a dN6primer and the superscript RT according to the manufacturers(Invitrogen) instructions. Fragments encoding VHH genes were amplifiedfrom this cDNA by PCR using specific primers as described previously(Conrad et al., 2001).

Using nested PCR, Pstl and BstEH were engineered at the start and theend of the VHH open reading frame, respectively. VHHs were cloned asPstl-BstEII fragments into the phage display vector pMESy4. For eachllama, a separate phage display library was constructed harboring therespective Nanobody repertoire as a geneIII fusion (Domanska et al.,2011). R:G complex specific nanobodies were enriched by two rounds ofbiopanning on i) the β2AR:Gs:BI167107 ternary complex embedded in ApoLbiotinylated high-density lipoprotein particles (rHDL, Whorton et al.,2007) or ii) on the BS2G cross-linked β2AR:Gs:BI167107 ternary complex.For the first biopanning strategy, biotinylated rHDL particlescontaining the β2AR:Gs:BI167107 ternary complex were immobilized on aneutravidin coated Maxisorp plate (Nunc) at 1 μg/well in 20 mM Hepes (pH8.0), 100 mM NaCl, 1 mM EDTA, 100 μM TCEP, and 100 nM BI167107. For thesecond biopanning strategy the BS2G cross-linked β2AR:Gs:BI167107ternary complex was solid phase coated on a Maxisorp plate at 1 μg/well.For each round of biopanning, 10¹¹ phage was added to immobilizedantigens and incubated for one to two hours. Next, non-bound phage wasremoved from the antigen containing wells and the wells were washed 14times with 20 mM HEPES, 100 mM NaCl, pH8 and finally incubated for 10minutes with 200 μL of 20 mM Hepes (pH 8.0), 100 mM NaCl, 1 mM EDTA, 100pM TCEP, and 100 nM BI167107 to remove aspecific phages. To elutecomplex specific phage, the wells were treated with trypsin, phage wasrecovered and used to infect exponentially growing TG1 cells(OD600±0.5).

From each enriched library, 48 colonies were randomly picked and grownin 1 ml 2xTY containing ampicillin and glucose. Cultures were inducedwith IPTG to induce the expression of the nanobodies and periplasmaticextracts containing a partially purified nanobody were prepared.Nanobodies contained in these periplasmic extracts were analyzed forbinding to the agonist:β2AR:Gs ternary complex by ELISA.

Nanobodies enriched on biotinylated rHDL particles containing theβ2AR:Gs:BI167107 ternary complex were analyzed by comparative ELISA onthe same complex immobilized on neutravidin coated Maxisorb platesversus empty rHDL particles. Nanobodies enriched on solid phase coatedBS2G cross-linked β2AR:Gs:BI167107 ternary complex were analyzed bycomparative ELISA on the same solid phase coated complex versusnon-coated wells. From colonies that scored positive in comparativeELISA, single clones were prepared, DNA was extracted and the sequencesof the encoded nanobody genes were analyzed using routine methods (aminoacid sequences shown in Tables 2 and 3). For Nb35, Nb36 and Nb37,binding to the β2AR:Gs:BI167107 ternary complex was further confirmed byanalytical gel filtration (FIGS. 3D, 3E, 3F, 3G).

Example 3 Nb35, Nb36 and Nb37 Bind to Gs and Prevent Dissociation of theComplex by GTPγS

To determine if the Nanobodies raised against the β2AR:Gs:BI167107ternary complex (Table 2) bind to the receptor or to Gs, we nextmonitored binding of these nanobodies in ELISA on purified receptoralone. All nanobodies from Table 2 scored negative in solid phase coated(Maxisorb, Nunc) agonist-bound β2AR-356 reconstituted at high densityinto phospholipid vesicles (Rasmussen et al., 2011). Nb80, a β2ARspecific nanobody (Rasmussen et al., 2011) scored positive in thisELISA. None of the β2AR:Gs:BI167107 binders described in Table 2 bindthe reconstituted receptor alone, indicating that they bind epitopescontained on Gs. Size exclusion chromatography shows that Nb35 and Nb37bind separate epitopes on the Gs heterotrimer to form a R:G:Nb35:Nb37complex (FIG. 3D). Similarly, Nb36 and Nb37 bind separate epitopes onthe Gs heterotrimer to form a R:G:Nb36:Nb37 complex (FIG. 3E).

GDP, GTP and non-hydrolyzable GTP analogs disrupt the β2AR:Gs complex(FIG. 3A), causing dissociation of GPCR G protein complexes in vitro andin vivo. The mutual effects of Nbs and the non-hydrolyzable GTP analogGTPγS on the integrity of the agonist:β2AR:Gs ternary complex wasanalyzed by analytical size exclusion chromatography in the presence andabsence of GTPγS. It was found that nanobodies 35, 36 and 37 protect theβ2AR:Gs:BI167107 complex from dissociation by GTPγS (FIGS. 3D, 3E and3G).

Example 4 Nanobody-Aided Crystallization of the β₂AR-Gs Complex

G protein-coupled receptors (GPCRs) are responsible for the majority ofcellular responses to hormones and neurotransmitters as well as thesenses of sight, olfaction and taste. The paradigm of GPCR signaling isthe activation of a heterotrimeric GTP binding protein (G protein) by anagonist-occupied receptor. In an effort to understand the structuralbasis for GPCR signaling, we crystallized the β₂AR-Gs complex to solveits structure by X-ray crystallography.

One challenge for crystallogenesis was to prepare a stable β₂AR:Gscomplex in detergent solution. The (β₂AR and Gs couple efficiently inlipid bilayers, but not in detergents used to solubilize and purifythese proteins (Example 1). It was found that a relatively stableβ₂AR:Gs complex could be prepared by mixing purified GDP-Gs(approximately 100 μM final concentration) with a molar excess ofpurified β₂AR bound to a high affinity agonist (BI167107; Rasmussen etal., 2011) in dodecylmaltoside solution. Apyrase, a non-selective purinepyrophosphatase, was added to hydrolyze GDP released from Gs on forminga complex with the β₂AR. The complex was subsequently purified bysequential antibody affinity chromatography and size exclusionchromatography. The stability of the complex was enhanced by exchangingit into a recently developed maltose neopentyl glycol detergent (NG-310,Anatrace) (Chae et al., 2010). This complex could be incubated at roomtemperature for 24 hours without any noticeable degradation; however,initial efforts to crystallize the complex using sparse matrix screensin detergent micelles, bicelles and lipidic cubic phase (LCP) failed.

To further assess the quality of the complex, the protein was analyzedby single particle electron microscopy (EM). The results confirmed thatthe complex was monodisperse (data not shown), but revealed otherpossible bottlenecks for obtaining diffraction of quality crystals.First, the detergent used to stabilize the complex formed a largemicelle, leaving little polar surface on the extracellular side of theβ₂AR:Gs complex for the formation of crystal lattice contacts.Therefore, we replaced the unstructured amino terminus of the β₂AR withT4 lysozyme (T4L). We previously used T4L to facilitate crystallogenesisof the inactive β₂AR by inserting T4L between the cytoplasmic ends oftransmembrane segments (TMs) 5 and 6 (Rosenbaum et al., 2007). Thisfusion protein (T4L-β₂AR) exhibited normal ligand binding and Gscoupling properties. Crystallization trials were carried out in LCPusing a modified monolein (7.7 MAG, provided by Martin Caffrey) designedto accommodate the large hydrophilic component of the T4L-β₂AR:Gscomplex (Misquitta et al., 2004). Although we were able to obtain smallcrystals that diffracted to 7 Å, we were unable to improve their qualitythrough the use of additives and other modifications.

Another possible problem for crystallogenesis revealed by singleparticle EM analysis was increased variability in the positioning of theα-helical component of the Gαs subunit. Gαs consists of two domains, theras-like GTPase domain (GαsRas), which interacts with the β₂AR and theGβ subunit, and the α-helical domain (GαsAH) (Sprang et al., 1997). Theinterface of the two Gαs subdomains forms the nucleotide-binding pocket(FIG. 1), and EM 2D averages and 3D reconstructions suggest that in theabsence of guanine nucleotide, GαsAH has a variable position relative tothe complex of T4L-β₂AR-GαsRAS-Gβγ (FIG. 1, Panel b).

In an effort to further facilitate crystallogenesis of the complex, wetried co-crystallization of the complex with nanobody 35. Nb35 protectsthe complex from dissociation by GTPγS, suggestive of a stabilizingGs:Nb interaction (FIG. 3A). BI-167107 bound T4L-β2AR:Gs complex andNb35 were mixed in 1:1.2 molar ratio (see FIGS. 6 and 7). The smallmolar excess of Nb35 was verified by analytical gel filtration (FIG. 7,Panel b). The mixture incubated for 1 hour at ROOM TEMPERATURE prior tomixing with 7.7 MAG (provided by Martin Caffrey) containing 10%cholesterol (C8667, Sigma) in 1:1 protein to lipid ratio (w/w) using thetwin-syringe mixing method reported previously (Caffrey 2009).Concentration of R:G:Nb complex in 7.7 MAG was approximately 25 mg/ml.The protein:lipid mixture was delivered through a LCP dispensing robot(Gryphon, Art Robbins Instruments) in 40 nl drops to either 24-well or96-well glass sandwich plates and overlaid en-bloc with 0.8 μlprecipitant solution. Multiple crystallization leads were initiallyidentified using in-house screens partly based on reagents from theStockOptions Salt kit (Hampton Research). Crystals for data collectionwere grown in 18 to 22% PEG 400, 100 mM MES pH 6.5 (FIG. 1, Panel c),350 to 450 mM potassium nitrate, 10 mM foscarnet (FIG. 3B), 1 mM TCEP,and 10 μM BI167107. Crystals reached full size within 3-4 days at 20° C.(FIG. 8) and were picked from a sponge-like mesophase and flash-frozenwithout additional cryo-protectant in liquid nitrogen.

Example 5 Nb35 Facilitates Crystal Formation of the R:G Complex

The BI-167107 bound T4L-β2AR:Gs:Nb35 complex crystallized in space groupP2₁, with a single complex in each asymmetric unit. FIG. 9, Panel a,shows the crystallographic packing interactions. FIG. 9, Panel b, showsthe structure of the complete complex including T4L and Nb35, and FIG.9, Panel c shows the β₂AR:Gs complex alone.

BI-167107 bound T4L-β2AR:Gs:Nb35 complexes are arrayed in alternatingaqueous and lipidic layers with lattice contacts formed almostexclusively between soluble components of the complex, leaving receptormolecules suspended between G protein layers and widely separated fromone another in the plane of the membrane. Extensive lattice contacts areformed among all the soluble proteins, likely accounting for the strongoverall diffraction and remarkably clear electron density for the Gprotein.

Nb35 and T4L facilitated crystal formation of the BI-167107 boundT4L-β2AR:Gs:Nb35 complex. Nb35 binds a conformational epitope on Gs andpacks at the interface of Gβ and Gα subunits withcomplementarity-determining region (CDR, defined according to IMTGnumbering; Lefranc, 2003) 1 interacting primarily with Gβ (FIG. 10A) anda long CDR3 loop interacting with both Gβ and Gα subunits (FIG. 10B).Some framework regions of Nb35 also interact with Gα from the samecomplex (FIG. 10C). Other framework regions from one complex interactwith Gα subunits from two adjacent complexes (FIGS. 11A and 11B),contributing considerably to the crystal contacts within the crystallattice. T4L forms relatively sparse interactions with the aminoterminus of the receptor, but packs against the amino terminus of the Gβsubunit of one complex, the carboxyl terminus of the Gγ subunit ofanother complex, and the Gαsubunit of yet another complex.

Example 6 Structure of the Active-State β₂AR

The β₂AR:Gs structure provides the first high-resolution insight intothe mechanism of signal transduction across the plasma membrane by aGPCR, and the structural basis for the functional properties of theternary complex. FIG. 12, Panel a, compares the structures of theagonist-bound receptor in the β₂AR:Gs complex and the inactivecarazolol-bound β₂AR. The largest difference between the inactive andactive structures is a 14 Å outward movement of TM6 when measured at theCα carbon of E268. There is a smaller outward movement and extension ofthe cytoplasmic end of the TM5 helix by 7 residues. A stretch of 26amino acids in the third intracellular loop (ICL3) is disordered.Another notable difference between inactive and active structures is thesecond intracellular loop (ICL2), which forms an extended loop in theinactive β₂AR structure and an α-helix in the β₂AR:Gs complex. Thishelix is also observed in the β₂AR-Nb80 structure (FIG. 12, Panel b);however, it may not be a feature that is unique to the active state,since it is also observed in the inactive structure of the highlyhomologous avian β₁AR (Warne et al., 2008).

The quality of the electron density maps for the β₂AR is highest at thisβ₂AR-GαsRas interface, and much weaker for the extracellular half,possibly due to the lack of crystal lattice contacts with theextracellular surface (FIG. 9, Panel a). As a result, we cannotconfidently model the high-affinity agonist (BI-167107) in theligand-binding pocket. However, the overall structure of the β₂AR in theT4L-β₂AR:Gs complex is very similar to our recent active-state structureof β₂AR stabilized by a G protein mimetic nanobody (Nb80). Thesestructures deviate primarily at the cytoplasmic ends of TMs 5 and 6(FIG. 12, Panel b), possibly due to the presence of T4L that replacesICL3 in the β₂AR-Nb80 structure. Nonetheless, the β₂AR-Nb80 complexexhibits the same high affinity for the agonist isoproterenol as doesthe β₂AR:Gs complex (Rasmussen et al., 2011), consistent with highstructural homology around the ligand binding pocket. The electrondensity maps for the β₂AR-Nb80 crystals provide a more reliable view ofthe conformational rearrangements of amino acids around theligand-binding pocket and between the ligand-binding pocket and theGs-coupling interface (Rasmussen et al., 2011).

FIG. 12, Panel c, shows the position of the highly conserved sequencemotifs including D/ERY and NPxxY in the β₂AR:Gs complex compared withthe β₂AR-Nb80 complex (see also FIG. 13). These conserved sequences havebeen proposed to be important for activation or for maintaining thereceptor in the inactive state (Hofmann et al., 2009). The positions ofthese amino acids are essentially identical in these two structuresdemonstrating that Nb80 is a very good G protein surrogate. Only Arg131differs between these two structures. In the β₂AR-Nb80 structure Arg131interacts with Nb80, whereas in the β₂AR:Gs structure Arg131 packsagainst Tyr391 of Gαs (FIG. 13).

The active state of the β₂AR is stabilized by extensive interactionswith (GαsRas) (FIG. 14). There are no direct interactions with Gβ or Gγsubunits. The total buried surface of the β₂AR-GαsRas interface is 2576A² (1300 A² for GαsRas and 1276 A² for the β₂AR). This interface isformed by ICL2, TM5 and TM6 of the β₂AR, and by α5-helix, the αN-β1junction, the top of the β3-strand, and the α4-helix of GαsRas (seeTable 6 for specific interactions). The β₂AR sequences involved in thisinteraction have been shown to play a role in G protein coupling;however, there is no clear consensus sequence for Gs-couplingspecificity when these segments are aligned with other GPCRs. Perhapsthis is not surprising considering that the β₂AR also couples to Gi andthat many GPCRs couple to more than one G protein isoform. Thestructural basis for G protein coupling specificity must thereforeinvolve more subtle features of the secondary and tertiary structure.Nevertheless, a noteworthy interaction involves Phe139, which is locatedat the beginning of the ICL2 helix and sits in a hydrophobic pocketformed by Gαs His41 at the beginning of the β1-strand, Val213 at thestart of the β3-strand and Phe376, Arg380 and Ile383 in the α5-helix(FIG. 14, Panel c). The β₂AR mutant F139A displays severely impairedcoupling to Gs (Moro et al., 1993). The residue corresponding to Phe139is a Phe or Leu on almost all Gs coupled receptors, but is more variablein GPCRs known to couple to other G proteins. Of interest, the ICL2helix is stabilized by an interaction between Asp130 of the conservedDRY sequence and Tyr141 in the middle of the ICL2 helix (FIG. 14, Panelc). Tyr141 has been shown to be a substrate for the insulin receptortyrosine kinase (Baltensperger et al., 1996); however, the functionalsignificance of this phosphorylation is currently unknown.

Example 7 Structure of Activated Gs

The most surprising observation in the β₂AR:Gs complex is the largedisplacement of the GαsAH relative to GαsRas (an approximately 180°rotation about the junction between the domains) (FIG. 15, Panel a). Inthe crystal structure of Gαs, the nucleotide-binding pocket is formed bythe interface between GαsRas and GαsAH. Guanine nucleotide bindingstabilizes the interaction between these two domains. The loss of thisstabilizing effect of guanine nucleotide binding is consistent with thehigh flexibility observed for GαsAH in single particle EM analysis ofthe detergent solubilized complex (data not shown). It is also inagreement with the increase in deuterium exchange at the interfacebetween these two domains upon formation of the complex (data notshown). Recently Hamm, Hubbell and colleagues, using double electronelectron resonance (DEER) spectroscopy, documented large (up to 20 Å)changes in distance between nitroxide probes positioned on the Ras andα-helical domains of Gi upon formation of a complex with light-activatedrhodopsin (Van Eps 2011). Therefore, it is perhaps not surprising thatGαsAH is displaced relative to GαsRas; however, its location in thiscrystal structure likely reflects the influence of crystal packinginteractions rather than a physiological conformation.

The conformational links between the β₂AR and the nucleotide-bindingpocket primarily involve the amino and carboxyl terminal helices of Gαs(FIG. 14). FIG. 15, Panel b, focuses on the region of GαsRAS thatundergoes the largest conformational change when comparing the structureof GαsRAS from the Gs-β₂AR complex with that from the Gαs-GTPγS complex(Sunahara et al., 1997). The largest difference is observed for the α5helix, which is displaced 6 Å towards the receptor and rotated as thecarboxyl terminal end projects into transmembrane core of the β₂AR.Associated with this movement, the β6-α5 loop, which interacts with theguanine ring in the Gαs-GTPγS structure, is displaced outward, away fromthe nucleotide-binding pocket (FIG. 15, Panels b-d). The movement of α5helix is also associated with changes in interactions between this helixand the β6 sheet, the αN-β1 loop, and the al helix. The β1 strand formsanother link between the β₂AR and the nucleotide-binding pocket. TheC-terminal end of this strand changes conformation around Gly47, andthere are further changes in the β1-α1 loop (P-loop) that coordinatesthe γ-phosphate in the GTP-bound form (FIG. 15, Panels b-d). Theobservations in the crystal structure are in agreement with deuteriumexchange experiments where there is enhanced deuterium exchange in theβ1 sheet and the amino terminal end of the α5 helix upon formation ofthe nucleotide-free β₂AR:Gs complex (data not shown).

The structure of a Gs heterotrimer has not been determined, so it is notpossible to directly compare the Gαs-Gβγ interface before and afterformation of the β₂AR:Gs complex. Based on the structure of the GDPbound Gi heterotrimer (Wall et al., 1995), we do not observe largechanges in interactions between GαsRAS and Gβγ upon formation of thecomplex with β₂AR. This is also consistent with deuterium exchangestudies (data not shown). It should be noted that Nb35 binds at theinterface between GαsRas and GP (FIG. 2, Panel b). Therefore, we cannotexclude the possibility that Nb35 may influence the relative orientationof the GαsRas-Gβγ interface in the crystal structure. However, singleparticle EM studies provide evidence that Nb35 does not disruptinteractions between GαsAH and GαsRas (data not shown).

Example 8 Nb35 and Nb37 Bind Different Epitopes on Gs and InhibitNucleotide Binding

To investigate the effect of nanobodies (Nb35 and Nb37) on Gs alone,nanobodies were added together with bodipy-GTPγS-FL and various Gsprotein preparations in 20 mM Tris-HCl, Ph 8.0, 3 mM MgCl₂, 1 mM DTT ina final volume of 200 μL. Samples containing a heterotrimeric Gs proteinalso included 0.1% DDM. Bodipy-GTPγS-FL is a stable fluorescent GTPanalog (λ_(ex)≅470 nm, λ_(em)≅515 nm). Its fluorescence intensityincreases upon G protein binding and Bodipy-GTPγS-FL can therefore beused for real-time measurements of nucleotide binding to G proteins(McEwen et al., 2001). Fluorescence was measured in a 96-well microtiterplate format on a M5 fluorescence plate reader (Molecular Precision).

In a first experiment (FIG. 16), increasing amounts of Nb37 wereincubated with 1 μM purified GαS and 100 nM Bodipy-GTPγS-FL and thefluorescence increase was measured on a short time scale (300 seconds)to minimize the accumulation of the hydrolysis product bodipy-phosphate(Jameson et al., 2005). The Gαs subunit of the heterotrimeric Gs proteinwas purified as described previously (Sunahara et al., 1997). From thisexperiment it appears that Nb37 blocks GTPγS binding to Gsα alone in adose dependent manner. These results also indicate that the bindingepitope of Nb37 is confined to the Gsα subunit of the heterotrimeric Gsprotein. In a similar experiment (FIG. 17), increasing amounts of Nb35were incubated with 1 μM purified GαS and 100 nM Bodipy-GTPγS-FL.Consistent with the observation that Nb35 binds an epitope composed ofelements of GαsRAS and Gβ (see Example 7), Nb35 has no effect on GTPγSbinding to the GαS subunit alone.

In another experiment (FIG. 18), increasing amounts of Nb35 wereincubated with 1 μM of the purified Gs αβγ heterotrimer and 100 nMBodipy-GTPγS-FL. This experiment indicates that Nb35 blocks GTPγSbinding to the free Gsαβγ heterotrimer in a dose dependent manner.

Example 9 Nb35 Stabilizes Other Agonist-GPCR-Gs Complexes

The Gs alpha subunit (or Gs protein) is a heterotrimeric G proteinsubunit that activates the cAMP-dependent pathway by activatingadenylate cyclase. The G protein-coupled receptors that couple to Gsinclude: 5-HT receptors types 5-HT4 and 5-HT7, ACTH receptor, Adenosinereceptor types A2a and A2b, Arginine vasopressin receptor 2,β-adrenergic receptors types β1, β2 and β3, Calcitonin receptor,Calcitonin gene-related peptide receptor, Corticotropin-releasinghormone receptor, Dopamine receptors D1-like family (D1 and D5),FSH-receptor, Gastric inhibitory polypeptide receptor, Glucagonreceptor, Glucagon-like peptide 1 receptor (GLP1-R), Histamine H2receptor, Luteinizing hormone/choriogonadotropin receptor, Melanocortinreceptor, Parathyroid hormone receptor 1, Prostaglandin receptor typesD2 and 12, Secretin receptor, Thyrotropin receptor, amongst others.

To determine if Nanobodies that bind to Gs in the β2AR:Gs:BI167107 alsostabilize other GPCR:Gs:agonist complexes, we prepared a complex of theArginine vasopressin receptor 2 (Accession number P30518; V2R_HUMAN) incomplex with Gs, Nb35 and Arginine vasopressin (AVP:NT4LV2R:Gs) anddemonstrated the stability of this complex in SEC. Arginine vasopressin(AVP), also known as vasopressin, argipressin or antidiuretic hormone,is a natural ligand that activates Arginine vasopressin receptor 2.

Formation of a stable complex (FIG. 19, Panel a) was accomplished bymixing His tagged Gs heterotrimer at approximately 90 μM concentrationwith AVP bound NT4LV2R (90 μM) and NB35 (100 μM) in 0.1 ml buffer (10 mMHEPES, pH 7.5, 100 mM NaCl, 0.1% DDM, 1 mM EDTA, 3 mM MgCl₂, 10 μM AVP)and incubated for 2 hours at room temperature. Next, the AVP:NT4LV2R:Gscomplex was purified in two successive affinity purification steps. Thepurification was monitored by SDS-PAGE (FIG. 19). First, the complex wasapplied on a Ni-NTA column after adding 300 al of 1% MNG in 10 mM HEPES,pH 7.5, 100 mM NaCl buffer onto the reaction mix. Following extensivewashing with buffer, the complex was eluted in 0.2% MNG, 10 mM HEPES, pH7.5, 100 mM NaCl AVP 10 μM with 200 mM imidazole. Next the complex wasapplied on a FLAG-tag affinity column, washed extensively in the samebuffer containing 0.01% MNG and eluted with the FLAG-peptide. Thisprocedure was monitored by SDS-PAGE (FIG. 19, Panel b) and shows that acomplex containing NT4LVTR, GαS, Gβ, Gγ and Nb35 can be purifiedaccordingly. This complex was further purified by SEC on a superdex200column in 10 mM HEPES, pH 7.5, 100 mM NaCl, 0.01% MNG, 1 mM EDTA, 3 mMMgCl₂, 1 μM AVP). This procedure was monitored by SDS-PAGE (FIG. 19,Panel c) and shows that a monodisperse complex containing NT4LVTR, GαS,Gβ, Gγ and Nb35 can be purified accordingly.

To confirm the stability of the AVP:NT4LV2R:Gs complex, we incubated thepurified sample for 24 hours on ice and reapplied it to SEC to confirmits monodisperse character and its MW (FIG. 20). As expected, an excessamount of the antagonist SR121463 (10 disrupts the AVP:NT4LV2R:Gscomplex.

Example 10 Improved Screening for Agonists or Positive AllostericModulators Using Nanobodies Stabilizing GPCR:G Protein Complexes

A Nanobody that selectively stabilizes a non-prominent conformer ofGPCRs will allow more efficient screening for ligands that selectivelyinteract with this particular low abundancy conformer. Besides,conformer selective Nanobodies could also be used to unmask allostericor hidden druggable sites or inversely mask undesired binding pocketsfor drug screening. In case the particular conformer is an active state,the identified ligands will have a high probability to behave asagonists, supported by in silico docking experiments described byCostanzi & Vilar (2011). Indeed, their results indicate that activatedstructures favor identification of agonists over antagonists, whereasinactive structures favor identification of receptor blockers overagonists.

Evidence is provided that Nb35 stabilizes the complex between theactivated β2AR and the G protein by binding the interface of Gαs and Gβγsubunits. An example of a screening assay to identify ligands thatselectively interact with the low abundant active state conformer ofβ2AR can be a radioligand assay using Nb35 to shift β2AR population moretowards its active state. Such radioligand assay can be executedsimilarly as the one described by Seifert and co-workers (1998) withminor modifications. We describe here an assay involving β2AR as thetarget of choice replacing β2AR by any other GPCR interacting with theGαs subunit, allow implementation of similar screening methods toidentify agonistic ligands against that particular GPCR. A smallmolecule (compound MW typically between 250 and 1000 Da) or even afragment based (compound MW typically <250 Da) library can be screenedto identify candidate agonists. The stabilization of the non-prominentconformer will increase the performance of the fragment library screensconsiderably, especially because the initial hits in fragment based drugscreening typically have a low potency/affinity. Nb35 will selectivelyincrease the affinity of those compounds that are specific for theselective, druggable conformer thus having a profound effect on theidentification of de novo fragments.

An appropriate amount (typically 10 μg) of human β2AR homogenizedmembrane extracts from HEK293T cells containing the membrane anchored Gprotein subunits are incubated in parallel with Nb35 or a non-relatedNanobody (which does not stabilize the active GPCR conformation) for 1hour at 30° C. in incubation buffer (50 mM Hepes pH 7.4, 1 mM CaCl2, 5mM MgCl2, 100 mM NaCl and 0.5% w/v BSA). Nanobodies are exogenouslysupplied in large molar excess (e.g., ≧1 μM) versus the adrenergicreceptor. Subsequently, the Nanobody-bound membranes are added to96-well plates containing library compounds and 2 nM of3H-dihydroalprenolol (DHA) antagonistic radioligand. The total volumeper well is adjusted with incubation buffer to 100 μl and the reactionmixture is further incubated for another hour at 30° C. Subsequently,membrane-bound radioligand is harvested using GF/C glass fiber 96-wellfilter plate (Perkin Elmer) presoaked in 0.3% polyethylenimine. Filterplates are washed with ice-cold wash buffer (50 mM Tris-HCl pH 7.4), anddried for 30 minutes at 50° C. After adding 25 μl of scintillation fluid(MICROSCINT™-O, Perkin Elmer), radioactivity (cpm) is measured in aWallac MicroBeta TriLux scintillation counter. Those library compoundsthat significantly decrease the cpm in presence of Nb35 while not usingthe non-related Nanobody are considered agonistic ligands. Candidateagonistic hits identified via the primary library screening will berescreened in a dose response manner. IC50 values of the % radioliganddisplacement curves for each candidate agonist in presence of Nb35 andthe non-related Nanobody will be calculated using the Graphpad Prismsoftware. To prove effective agonism of the identified de novocompounds, the dose dependent effect of these compounds in a cellularβ2AR signaling assay will be evaluated. One example of such assay relieson the detection of secondary messenger molecules such as cAMP after Gαsmediated signaling (e.g., HitHunter cAMP assay technology, DiscoverX).Instead of using membrane extracts and exogenously applied Nb35, theradioligand assay could be performed on a β2AR expressing cell lineco-transfected with Nb35 as an intrabody (or derived membranes) in orderto shift the β2AR population to its active state. Alternatively,recombinant G protein and β2AR can be used to stabilize via Nb35 theactive state of the β2AR.

Material and Methods to the Examples

Expression and Purification of β2AR, Gs Heterotrimer, and Nanobody-35

An N-terminally fused T4 lysozyme-β2AR construct truncated in position365 (T4L-β2AR, described in detail below) was expressed in Sf-9 insectcell cultures infected with recombinant baculovirus (BestBac, ExpressionSystems), and solubilized in n-Dodecyl-β-D-maltoside (DDM) according tomethods described previously (Kobilka et al., 1995) (see FIG. 6 forpurification overview). A β2AR construct truncated after residue 365(β2AR-365; SEQ ID NO:55) was used for the majority of the analyticalexperiments and for deuterium exchange experiments. M1 Flag affinitychromatography (Sigma) served as the initial purification step followedby alprenolol-Sepharose chromatography for selection of functionalreceptor. A subsequent M1 Flag affinity chromatography step was used toexchange receptor-bound alprenolol for high-affinity agonist BI-167107.The agonist-bound receptor was eluted, dialyzed against buffer (20 mMHEPES pH7.5, 100 mM NaCl, 0.1% DDM and 10 μM BI-167107), treated withlambda phosphatase (New England Biolabs), and concentrated toapproximately 50 mg ml⁻¹ with a 50 kDa molecular weight cut off (MWCO)Millipore concentrator. Prior to spin concentration the β2AR-365construct, but not T4L-β2AR, was treated with PNGaseF (New EnglandBiolabs) to remove amino-terminal N-linked glycosylation. The purifiedreceptor was routinely analyzed by SDS-PAGE/Coomassie brilliant bluestaining (see FIG. 5, Panel a).

Bovine Gα_(s) short, rat Gβ₁ fused to a His₆ tag, and rat Gγ₂ (see Table5) were expressed in High 5 insects cells (Invitrogen) grown in InsectXpress serum-free media (Lonza). Cultures were grown to a density of 1.5million cells per ml and then infected with three separate Autographacalifornica nuclear polyhedrosis virus each containing the gene for oneof the G protein subunits at a 1:1 multiplicity of infection (theviruses were a generous gift from Dr. Alfred Gilman). After 40-48 hoursof incubation the infected cells were harvested by centrifugation andresuspended in 75 ml lysis buffer (50 mM HEPES pH 8.0, 65 mM NaCl, 1.1mM MgCl₂, 1 mM EDTA, 1×PTT (35 μg/ml phenylmethanesulfonyl fluoride, 32μg/ml tosyl phenylalanyl chloromethyl ketone, 32 μg/ml tosyl lysylchloromethyl ketone), 1×LS (3.2 μg/ml leupeptin and 3.2 μg/ml soybeantrypsin inhibitor), 5 mM 13-ME, and 10 μM GDP) per liter of culturevolume. The suspension was pressurized with 600 psig N₂ for 40 minutesin a nitrogen cavitation bomb (Parr Instrument Company). Afterdepressurization, the lysate was centrifuged to remove nuclei andunlysed cells, and then ultracentrifuged at 180,000×g for 40 minutes.The pelleted membranes were resuspended in 30 ml wash buffer (50 mMHEPES pH 8.0, 50 mM NaCl, 100 μM MgCl₂, 1×PTT, 1×LS, 5 mM 13-ME, 10 μMGDP) per liter culture volume using a Dounce homogenizer and centrifugedagain at 180,000×g for 40 minutes. The washed pellet was resuspended ina minimal volume of wash buffer and flash frozen with liquid nitrogen.

The frozen membranes were thawed and diluted to a total proteinconcentration of 5 mg/ml with fresh wash buffer. Sodium cholatedetergent was added to the suspension at a final concentration of 1.0%,MgCl₂ was added to a final concentration of 5 mM, and 0.05 mg ofpurified protein phosphatase 5 (prepared in house) was added per literof culture volume. The sample was stirred on ice for 40 minutes, andthen centrifuged at 180,000×g for 40 minutes to remove insolublederbies. The supernatant was diluted 5-fold with Ni-NTA load buffer (20mM HEPES pH 8.0, 363 mM NaCl, 1.25 mM MgCl₂, 6.25 mM imidazole, 0.2%Anzergent 3-12, 1×PTT, 1×LS, 5 mM β-ME, 10 μM GDP), taking care to addthe buffer slowly to avoid dropping the cholate concentration below itscritical micelle concentration too quickly. 3 ml of Ni-NTA resin(Qiagen) pre-equilibrated in Ni-NTA wash buffer 1 (20 mM HEPES pH 8.0,300 mM NaCl, 2 mM MgCl₂, 5 mM imidazole, 0.2% Cholate, 0.15% Anzergent3-12, 1×PTT, 1×LS, 5 mM β-ME, 10 μM GDP) per liter culture volume wasadded and the sample was stirred on ice for 20 minutes. The resin wascollected into a gravity column and washed with 4× column volumes ofNi-NTA wash buffer 1, Ni-NTA wash buffer 2 (20 mM HEPES pH 8.0, 50 mMNaCl, 1 mM MgCl₂, 10 mM imidazole, 0.15% Anzergent 3-12, 0.1% DDM,1×PTT, 1×LS, 5 mM β-ME, 10 μM GDP), and Ni-NTA wash buffer 3 (20 mMHEPES pH 8.0, 50 mM NaCl, 1 mM MgCl₂, 5 mM imidazole, 0.1% DDM, 1×PTT,1×LS, 5 mM β-ME, 10 μM GDP). The protein was eluted with Ni-NTA elutionbuffer (20 mM HEPES pH 8.0, 40 mM NaCl, 1 mM MgCl₂, 200 mM imidazole,0.1% DDM, 1×PTT, 1×LS, 5 mM β-ME, 10 μM GDP). Protein-containingfractions were pooled and MnCl2 was added to a final concentration of100 μM. 50 μg of purified lambda protein phosphatase (prepared in house)was added per liter of culture volume and the elute was incubated on icewith stirring for 30 minutes. The elute was passed through a 0.22 umfilter and loaded directly onto a MonoQ HR 16/10 column (GE Healthcare)equilibrated in MonoQ buffer A (20 mM HEPES pH 8.0, 50 mM NaCl, 100 μMMgCl₂, 0.1% DDM, 5 mM 13-ME, 1×PTT). The column was washed with 150 mlbuffer A at 5 ml/min and bound proteins were eluted over 350 ml with alinear gradient up to 28% MonoQ buffer B (same as buffer A except with 1M NaCl). Fractions were collected in tubes spotted with enough GDP tomake a final concentration of 10 μM. The Gs containing fractions wereconcentrated to 2 ml using a stirred ultrafiltration cell with a 10 kDaNMWL regenerated cellulose membrane (Millipore). The concentrated samplewas run on a Superdex 200 prep grade XK 16/70 column (GE Healthcare)equilibrated in S200 buffer (20 mM HEPES pH 8.0, 100 mM NaCl, 1.1 mMMgCl₂, 1 mM EDTA, 0.012% DDM, 100 μM TCEP, 2 μM GDP). The fractionscontaining pure Gs were pooled, glycerol was added to 10% finalconcentration, and then the protein was concentrated to at least 10mg/ml using a 30 kDa MWCO regenerated cellulose Amicon centrifugalultrafiltration device. The concentrated sample was then aliquoted,flash frozen, and stored at −80°. A typical yield of final, purified Gsheterotrimer from 8 liters of cell culture volume was 6 mg.

Nanobody-35 (Nb35) (SEQ ID NO: 1) was expressed in the periplasm ofEscherichia coli strain WK6, extracted, and purified by nickel affinitychromatography according to previously described methods (Rasmussen etal., 2011) followed by ion-exchange chromatography (FIG. 7a ) using aMono S 10/100 GL column (GE Healthcare). Selected Nb35 fractions weredialysis against buffer (10 mM HEPES pH 7.5, 100 mM NaCl) andconcentrated to approximately 65 mg ml⁻¹ with a 10 kDa MWCO Milliporeconcentrator.

Protein Engineering

To increase the probability of obtaining crystals of the R:G complex weset out to increase the polar surface area of the receptorsextracellular surface using two strategies. The approach was to replacethe flexible and presumably unstructured N-terminus with the globularprotein T4 lysozyme (T4L) used previously to crystallize and solve thecarazolol bound receptor (Rosenbaum et al., 2007). The construct usedhere (T4L-β2AR) contained the cleavable signal sequence followed by theM1 Flag epitope (DYKDDDDA; SEQ ID NO:70), the TEV protease recognitionsequence (ENLYFQG; SEQ ID NO:71), bacteriophage T4 lysozyme from N2through Y161 including C54T and C97A mutations, and a two residuealanine linker fused to the human β₂AR sequence D29 through G365(T4L-β2AR fusion construct defined by SEQ ID NO:69). The PNGaseFinaccessible glycosylation site of the β₂AR at N187 was mutated to Glu.M96 and M98 in the first extracellular loop were each replaced by Thr toincrease the otherwise low expression level of T4L-β2AR. The threoninemutations did not affect ligand binding affinity for³H-dihydro-alprenolol, but caused a small, approximately two-folddecrease in affinity for isoproterenol (data not shown). Note that thewild type reference β2AR that is used here is defined by SEQ ID NO:72.

Microcrystallography Data Collection and Processing.

Data collection was performed at the Advanced Photon Source beamline 23ID-B. Hundreds of crystals were screened, and a final dataset wascompiled using diffraction wedges of typically 10 degrees from 20strongly diffracting crystals. All data reduction was performed usingHKL2000 (Otwinowski et al., 1997). Although in many cases diffraction tobeyond 3 Å was seen in initial frames, radiation damage and anisotropicdiffraction resulted in low completeness in higher resolution shells.Analysis of the final dataset by the UCLA diffraction anisotropy server(Strong et al., 2006) indicated that diffraction along the a* reciprocalaxis was superior to that in other directions. On the basis of an F/sigFcutoff of 3 along each reciprocal space axis, reflections were subjectedto an anisotropic truncation with resolution limits of 2.9, 3.2, and 3.2Angstroms along a*, b*, and c* prior to use in refinement. Due to thelow completeness in high-resolution shells, we report this structure toan overall resolution of only 3.3 Å, although it should be noted thatsome diffraction data to 2.9 Å was included during refinement and mapcalculation.

Structure Solution and Refinement

The structure was solved by molecular replacement using Phaser (McCoy etal., 2007a, b). The order of the molecular replacement search was foundto be critical in solving the structure. In the order used, searchmodels were: the structure of beta and gamma subunits from the structureof a Gi heterotrimeric G protein (PDB ID: 1GP2), Gs alpha ras domain(PDB ID: 1AZT), active-state beta2 adrenergic receptor (PDB ID: 3P0G),beta2 binding nanobody (PDB ID: 3P0G), T4 lysozyme (PDB ID: 2RH1), Gsalpha helical domain (PDB ID: 1AZT). Following the determination of theinitial structure by molecular replacement, rigid body refinement andsimulated annealing were performed in Phenix (Afonine et al., 2005) andBUSTER (Blanc et al., 2004), followed by restrained refinement andmanual rebuilding in Coot (Emsley et al., 2004). After iterativerefinement and manual adjustments, the structure was refined in CNSusing the DEN method. Although the resolution of this structure exceedsthat for which DEN is typically most useful, the presence of severalpoorly resolved regions indicated that the incorporation of additionalinformation to guide refinement could provide better results. The DENreference models used were those indicated above as molecularreplacement search models, with the exception of NB35, which was wellordered and for which no higher resolution structure is available.Figures were prepared using PyMOL (The PyMOL Molecular Graphics System,Version 1.3, Schrödinger, LLC.). Refinement statistics are given inTable 7.

Binding

Membranes expressing the β2AR or the β₂AR-Gspeptide fusion were preparedfrom baculovirus-infected Sf9 cells and ³H-dihydroalprenolol (³H-DHA)binding performed as previously described (Swaminath et al., 2002). Forcompetition binding, membranes were incubated with ³H-DHA (1.1 nM final)and increasing concentrations of (−)-isoproterenol (ISO) for 1 hourbefore harvesting onto GF/B filters. Competition data were fitted to atwo-site binding model and ISO high and low Kis and fractions calculatedusing GraphPad prism.

Purification of NT4LV2R

The N-terminally fused T4L V2R construct (NT4L-V2R; SEQ ID NO:73) wasexpressed in Sf9 cells using the baculovirus system (PfastBac). Cellswere infected at a density of 4×10⁶ cells per ml and culture flasks wereshaken at 27° C. for 48 hours. After harvesting, cells were lysed byosmotic shock in a buffer comprised of 10 mM Tris-HCl pH 7.5, 1 mM EDTA,1 μM Tolvaptan (Sigma) and 2 mg ml⁻¹ iodoacetamide to block reactivecysteines. Extraction of NT4L-V2R from Sf9 membranes was done with aDounce homogenizer in a solubilization buffer comprised of 0.5% dodecylmaltoside (DDM), 0.3% Cholate, 0.03% cholesterol hemisuccinate (CHS), 20mM HEPES pH 7.5, 0.5 M NaCl, 30% v/v glycerol, 2 mgml⁻¹ iodoacetamideand 1 μM Tolvaptan. After centrifugation, nickel-NTA agarose was addedto the supernatant, stirred for 2 hours, and then washed in batch with100 g spins for 5 minutes each with a washing buffer of 0.1% DDM, 0.03%Cholate, 0.01% CHS, 20 mM HEPES pH 7.5 and 0.5 M NaCl. The resin waspoured into a glass column and bound receptor was eluted in washingbuffer supplemented with 300 mM imidazole. We used anti-Flag M1 affinityresin to purify NT4L-V2R further and to exchange the ligand with theagonist AVP. Ni-NTA resin eluate was loaded onto anti-Flag M1 resin andwashed extensively in the presence of 10 μM AVP. Receptor was theneluted from the anti-Flag M1 affinity resin with 0.2 mgml⁻¹ Flag peptideand 2 mM EDTA in the presence of 1 μM AVP and concentrated using a 100kDa MWCO concentrator.

TABLE 2  List of nanobodies Nanobody Nanobody SEQ reference short IDSequence number notation NO:(including C-terminal Histidine tag and EPEA tag) CA4435 Nb35 1QVQLQESGGGLVQPGGSLRLSCAASGFTFSNYKMNWVRQAPGKGLEWVSDISQSGASISYTGSVKGRFTISRDNAKNTLYLQMNSLKPEDTAVYYCARCPAPFTRDCFDVTSTTYAY RGQGTQVTVSSHHHHHHEPEA CA4433Nb33 2 QVQLQESGGGLVQPGGSLRLSCAASGFTFSNYVMNWVRQAPGKGLEWVSDISNGGGTTSYASSVKGRFTISRDNAKNTLYLQMNGLKPADTAVYYCARCPAPFTNDCMDITSTTYA YRGQGTQVTVSSHHHHHHEPEA CA4436Nb36 3 QVQLQESGGGSVQAGGSLRLSCTVSGTIFSVTVMGWYRQAPGKQRELVAGFTNTRNTNYVDSVKGRFTISKDSAKNTMYLQMNSLKPEDTAVYYCNVRRWGGTNWNDYWGQGTQ VTVSSHHHHHHEPEA CA4437 Nb37 4QVQLQESGGGFVQAGGSLRLSCAASGSIFSKNTMAWFRQAPGKERELVAASPTGGSTAYKDSVKGRFTISRDSAKNTVLLQMNVLKPEDTAVYYCHLRQNNRGSWFHYWGQGTQVT VSSHHHHHHEPEA CA4440 Nb40 5QVQLQESGGGLVQAGGSLRLSCAVSGTIFDITPMGWYRQTPGKQREVVADLTSRGTTNYADSVKGRFTISRDNAKKMLYLQMNSLKSDDTGVYYCNVKRWGGIGWNDYWGQGTQ VTVSSHHHHHHEPEA CA4441 Nb41 6QVQLQESGGGLVQSGGSLRLSCVASGFRFSNFPMMWVRQAPGKGLEWVSLISIGGSTTNYADSVKGRFTISRDNAKNTLFLQMNSLKPEDTAVYYCAKYLGRLVPPTTEGQGTQVTV SSHHHHHHEPEA

TABLE 3  Combinations of FRs and CDRs of nanobodies Nanobody Nanobodyreference short number notation FR1 CDR1 FR2 CDR2 FR3 CDR3 FR4 CA4435NB35 QVQLQESGG GFTFSNYK MNWVRQAP ISQSGASI SYTGSVKGRF ARCPAPFTRDRGQGTQVTV GLVQPGGSL (SEQ GKGLEWVSD (SEQ ID TISRDNAKNTL CFDVTSTTYA SSRLSCAAS ID NO: 13) (SEQ ID NO: 25) YLQMNSLKPE Y (SEQ ID (SEQ ID NO: 7)NO: 19) DTAVYYC (SEQ ID NO: 43) (SEQ ID NO: 31) NO: 37) CA4433 Nb33QVQLQESGG GFTFSNYV MNWVRQAP ISNGGGTT SYASSVKGRF ARCPAPFTND RGQGTQVTVGLVQPGGSL (SEQ GKGLEWVSD (SEQ TISRDNAKNTL CMDITSTTYA SS RLSCAASID NO: 14) (SEQ ID NO: 26) YLQMNGLKPA Y (SEQ (SEQ ID NO: 8) ID NO: 20)DTAVYYC (SEQ ID ID NO: 44) (SEQ ID NO: 32)  NO: 38) CA4436 Nb36QVQLQESGG GTIFSVTV MGWYRQAP FTNTRNT NYVDSVKGRF NVRRWGGTN WGQGTQVTVGSVQAGGSL (SEQ GKQRELVAG (SEQ TISKDSAKNT WNDY SS RLSCTVS ID NO: 15) (SEQID NO: 27) MYLQMNSLKP (SEQ ID (SEQ (SEQ ID NO: 9) ID NO: 21) EDTAVYYCNO: 39) ID NO: 45) (SEQ ID NO: 33) CA4437 Nb37 QVQLQESGG GSIFSKNTMAWFRQAPG SPTGGST AYKDSVKGRF HLRQNNRGS WGQGTQVTV GFVQAGGSL (SEQ KERELVAA(SEQ TISRDSAKNTV WFHY SS RLSCAAS ID NO: 16) (SEQ ID NO: 28) LLQMNVLKPE(SEQ (SEQ (SEQ ID NO: 22) DTAVYYC ID NO: 40) ID NO: 46) ID NO: 10)(SEQ ID NO: 34) CA4440 Nb40 QVQLQESGG GTIFDITP MGWYRQTPG LTSRGTTNYADSVKGRF NVKRWGGIG WGQGTQVTV GLVQAGGSL (SEQ KQREVVAD (SEQ TISRDNAKKMWNDY SS (SEQ RLSCAVS ID NO: 17) (SEQ ID NO: 29) LYLQMNSLKS (SEQID NO: 47) (SEQ ID NO: 23) DDTGVYYC ID NO: 41) ID NO: 11)(SEQ ID NO: 35) CA4441 Nb41 QVQLQESGG GFRFSNFP MMWVRQAP ISIGGSTTNYADSVKGRF AKYLGRLVP EGQGTQVTV GLVQSGGSL (SEQ GKGLEWVSL (SEQ TISRDNAKNTLPTT SS (SEQ RLSCVAS ID NO: 18) (SEQ ID NO: 30) FLQMNSLKPE (SEQID NO: 48) (SEQ ID NO: 24) DTAVYYC ID NO: 42) ID NO: 12) (SEQ ID NO: 36)

TABLE 4  Nucleic acid sequences of nanobodies Nanobody Nanobody SEQNucleotide sequence of the nanobody reference short ID(including nucleotide sequences of His number notation NO:tag and EPEA tag, which are underlined) CA4435 Nb35 49CAGGTGCAGCTGCAGGAGTCTGGAGGAGGCTTGGTGCAGCCTGGGGGGTCTCTGAGACTCTCCTGTGCGGCCTCTGGATTCACCTTCAGCAATTATAAAATGAACTGGGTCCGCCAGGCTCCAGGAAAGGGGCTCGAGTGGGTCTCAGATATTTCTCAGAGTGGTGCTAGCATAAGTTACACAGGCTCCGTGAAGGGCCGATTCACCATCTCCAGAGACAACGCCAAGAACACGCTGTATCTACAAATGAACAGCCTGAAGCCTGAGGACACGGCCGTCTATTACTGTGCCAGATGTCCGGCCCCATTCACGAGAGATTGTTTTGACGTGACTAGTACCACGTATGCCTACAGGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACCA TCACCATCACGAACCTGAAGCCTAG CA4433Nb33 50 CAGGTGCAGCTGCAGGAGTCTGGAGGGGGCTTGGTGCAGCCTGGGGGGTCTCTGAGACTCTCCTGTGCAGCCTCTGGATTCACTTTCAGTAACTATGTCATGAACTGGGTCCGCCAGGCTCCAGGAAAGGGGCTCGAGTGGGTCTCAGATATTTCTAATGGCGGTGGTACCACAAGTTATGCAAGCTCCGTGAAGGGCCGATTCACCATCTCCAGAGACAACGCCAAGAACACGCTGTATCTGCAAATGAACGGCCTGAAGCCTGCGGACACGGCCGTCTATTACTGTGCAAGATGTCCGGCCCCATTCACGAACGATTGTATGGACATAACTAGTACCACGTATGCCTACAGGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACC ATCACCATCACGAACCTGAAGCCTAGCA4436 Nb36 51 CAGGTGCAGCTGCAGGAGTCTGGAGGAGGCTCGGTGCAGGCTGGGGGGTCTCTGAGACTCTCCTGTACAGTCTCTGGAACCATCTTCAGTGTCACTGTCATGGGCTGGTACCGCCAGGCTCCAGGGAAGCAGCGCGAGTTGGTCGCAGGTTTTACTAATACTAGAAACACAAACTATGTAGACTCCGTGAAGGGCCGCTTCACCATCTCCAAAGACAGCGCCAAGAACACGATGTATCTACAAATGAACAGCCTGAAACCTGAGGACACAGCCGTCTATTACTGTAATGTACGTCGGTGGGGCGGTACGAATTGGAATGACTACTGGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACCATCACCATCACGAACCTGAAGCCTAG CA4437 Nb37 52CAGGTGCAGCTGCAGGAGTCTGGAGGGGGCTTCGTGCAGGCTGGGGGGTCTCTGAGACTCTCCTGTGCAGCCTCTGGAAGCATCTTCAGTAAGAATACCATGGCCTGGTTCCGCCAGGCTCCAGGGAAGGAGCGAGAGTTGGTCGCAGCTAGTCCTACGGGTGGTAGCACAGCGTATAAAGACTCCGTGAAGGGCCGATTCACCATCTCCAGAGACAGCGCCAAGAACACGGTGTTGCTGCAAATGAACGTCCTGAAACCTGAGGATACTGCCGTCTATTACTGTCATCTACGTCAAAATAACCGTGGTTCTTGGTTCCACTACTGGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACCATCACCATCACGAACCTGAAGCCTAG CA4440 Nb40 53CAGGTGCAGCTGCAGGAGTCTGGGGGAGGCTTGGTGCAGGCTGGGGGGTCGCTGAGACTCTCTTGTGCAGTCTCTGGTACGATCTTCGATATCACTCCCATGGGCTGGTACCGCCAGACTCCAGGGAAGCAGCGCGAAGTGGTCGCAGATCTTACTAGTCGCGGTACCACAAATTACGCAGACTCCGTGAAGGGCCGGTTCACCATCTCCAGAGACAACGCCAAGAAAATGTTGTATCTGCAAATGAACAGCCTGAAATCTGACGACACAGGCGTGTATTACTGTAACGTGAAACGGTGGGGAGGTATTGGCTGGAACGACTACTGGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACCATCACCATCACGAACCTGAAGCCTAG CA4441 Nb41 54CAGGTGCAGCTGCAGGAGTCTGGAGGAGGCTTGGTGCAGTCTGGGGGGTCTCTGAGACTCTCCTGTGTAGCCTCTGGATTCAGATTCAGTAACTTTCCTATGATGTGGGTCCGCCAGGCCCCAGGAAAGGGGCTCGAGTGGGTCTCGCTGATTAGCATTGGTGGTAGTACCACGAATTATGCGGACTCCGTGAAGGGCCGATTCACCATCTCCAGAGACAACGCCAAGAACACGCTGTTTCTGCAAATGAACAGCCTGAAACCTGAGGACACGGCCGTGTATTACTGTGCAAAATATCTTGGTCGGCTGGTCCCACCGACTACTGAGGGCCAGGGGACCCAGGTCACCGTCTCCTCACACCACCATCACCATCACGAACCTGAAGCCTAG

TABLE 5  Examples of isoforms of G protein subunits Accession number(SEQ ID Protein/subunit NO:) Isoform AA sequence human Gαs short P63092GNAS2_HUMAN MGCLGNSKTEDQRNEEKAQREANKKIEKQL (56)QKDKQVYRATHRLLLLGAGESGKSTIVKQM RILHVNGFNGDSEKATKVQDIKNNLKEAIETIVAAMSNLVPPVELANPENQFRVDYILSVMNV PDFDFPPEFYEHAKALWEDEGVRACYERSNEYQLIDCAQYFLDKIDVIKQADYVPSDQDLLRC RVLTSGIFETKFQVDKVNFHMFDVGGQRDERRKWIQCFNDVTAIIFVVASSSYNMVIREDNQT NRLQEALNLFKSIWNNRWLRTISVILFLNKQDLLAEKVLAGKSKIEDYFPEFARYTTPEDATPE PGEDPRVTRAKYFIRDEFLRISTASGDGRHYCYPHFTCAVDTENIRRVFNDCRDIIQRMHLRQY ELL human Gαi P63096 GNAI1_HUMANMGCTLSAEDKAAVERSKMIDRNLREDGEKA (57) AREVKLLLLGAGESGKSTIVKQMKIIHEAGYSEEECKQYKAVVYSNTIQSIIAIIRAMGRLKIDF GDSARADDARQLFVLAGAAEEGFMTAELAGVIKRLWKDSGVQACFNRSREYQLNDSAAYY LNDLDRIAQPNYIPTQQDVLRTRVKTTGIVETHFTFKDLHFKMFDVGGQRSERKKWIHCFEGV TAIIFCVALSDYDLVLAEDEEMNRMHESMKLFDSICNNKWFTDTSIILFLNKKDLFEEKIKKSP LTICYPEYAGSNTYEEAAAYIQCQFEDLNKRKDTKEIYTHFTCATDTKNVQFVFDAVTDVIIKN NLKDCGLF human Gαt P11488 GNAT1_HUMANMGAGASAEEKHSRELEKKLKEDAEKDARTV (58) KLLLLGAGESGKSTIVKQMKIIHQDGYSLEECLEFIABYGNTLQSILAIVRAMTTLNIQYGDSA RQDDARKLMHMADTIEEGTMPKEMSDIIQRLWKDSGIQACFERASEYQLNDSAGYYLSDLER LVTPGYVPTEQDVLRSRVKTTGIIETQFSFKDLNFRMFDVGGQRSERKKWIHCFEGVTCIIFIAA LSAYDMVLVEDDEVNRMHESLHLFNSICNHRYFATTSIVLFLNKKDVFFEKIKKAHLSICFPDY DGPNTYEDAGNYIKVQFLELNMRRDVKEIYSHMTCATDTQNVKFVFDAVTDIIIKENLKDCG LF Bovine Gαs short P04896 GNAS2_BOVINMGCLGNSKTEDQRNEEKAQREANKKIEKQL (59) QKDKQVYRATHRLLLLGAGESGKSTIVKQMRILHVNGFNGEGGEEDPQAARSNSDGEKATK VQDIKNNLKEAIETIVAAMSNLVPPVELANPENQFRVDYILSVMNVPDFDFPPEFYEHAKALW EDEGVRACYERSNEYQLIDCAQYFLDKIDVIKQDDYVPSDQDLLRCRVLTSGIFETKFQVDKV NFHMFDVGGQRDERRKWIQCFNDVTAIIFVVASSSYNMVIREDNQTNRLQEALNLFKSIWNN RWLRTISVILFLNKQDLLAEKVLAGKSKIEDYFPEFARYTTPEDATPEPGEDPRVTRAKYFIRD EFLRISTASGDGRHYCYPHFTCAVDTENIRRVFNDCRDIIQRMHLRQYELL Rat Gαs short P63095 GNAS2_RATMGCLGNSKTEDQRNEEKAQREANKKIEKQL (60) QKDKQVYRATHRLLLLGAGESGKSTIVKQMRILHVNGFNGEGGEEDPQAARSNSDGEKATK VQDIKNNLKEAIETIVAAMSNLVPPVELANPENQFRVDYILSVMNVPNFDFPPEFYEHAKALW EDEGVRACYERSNEYQLIDCAQYFLDKIDVIKQADYVPSDQDLLRCRVLTSGIFETKFQVDKV NFHMFDVGGQRDERRKWIQCFNDVTAIIFVVASSSYNMVIREDNQTNRLQEALNLFKSIWNN RWLRTISVILFLNKQDLLAEKVLAGKSKIEDYFPEFARYTTPEDATPEPGEDPRVTRAKYFIRD EFLRISTASGDGRHYCYPHFTCAVDTENIRRVFNDCRDIIQRMHLRQYELL Mouse Gαs short P63094 GNAS2_MOUSEMGCLGNSKTEDQRNEEKAQREANKKIEKQL (61) QKDKQVYRATHRLLLLGAGESGKSTIVKQMRILHVNGFNGEGGEEDPQAARSNSDGEKATK VQDIKNNLKEAIETIVAAMSNLVPPVELANPENQFRVDYILSVMNVPNFDFPPEFYEHAKALW EDEGVRACYERSNEYQLIDCAQYFLDKIDVIKQADYVPSDQDLLRCRVLTSGIFETKFQVDKV NFHMFDVGGQRDERRKWIQCFNDVTAIIFVVASSSYNMVIREDNQTNRLQEALNLFKSIWNN RWLRTISVILFLNKQDLLAEKVLAGKSKIEDYFPEFARYTTPEDATPEPGEDPRVTRAKYFIRD EFLRISTASGDGRHYCYPHFTCAVDTENIRRVFNDCRDIIQRMHLRQYELL Bovine Gβ P62871 GBB1_BOVINMSELDQLRQEAEQLKNQIRDARKACADATLS (62) QITNNIDPVGRIQMRTRRTLRGHLAKIYAMHWGTDSRLLVSASQDGKLIIWDSYTTNKVHAIP LRSSWVMTCAYAPSGNYVACGGLDNICSIYNLKTREGNVRVSRELAGHTGYLSCCRFLDDNQ IVTSSGDTTCALWDIETGQQTTTFTGHTGDVMSLSLAPDTRLFVSGACDASAKLWDVREGM CRQTFTGHESDINAICFFPNGNAFATGSDDATCRLFDLRADQELMTYSHDNIICGITSVSFSKSG RLLLAGYDDFNCNVWDALKADRAGVLAGHDNRVSCLGVTDDGMAVATGSWDSFLKIWN Human Gβ P62873 GBB1_HUMANMSELDQLRQEAEQLKNQIRDARKACADATLS (63) QITNNIDPVGRIQMRTRRTLRGHLAKIYAMHWGTDSRLLVSASQDGKLIIWDSYTTNKVHAIP LRSSWVMTCAYAPSGNYVACGGLDNICSIYNLKTREGNVRVSRELAGHTGYLSCCRFLDDNQ IVTSSGDTTCALWDIETGQQTTTFTGHTGDVMSLSLAPDTRLFVSGACDASAKLWDVREGM CRQTFTGHESDINAICFFPNGNAFATGSDDATCRLFDLRADQELMTYSHDNIICGITSVSFSKSG RLLLAGYDDFNCNVWDALKADRAGVLAGHDNRVSCLGVTDDGMAVATGSWDSFLKIWN Rat Gβ P54311 GBB1_RATMSELDQLRQEAEQLKNQIRDARKACADATLS (64) QITNNIDPVGRIQMRTRRTLRGHLAKIYAMHWGTDSRLLVSASQDGKLIIWDSYTTNKVHAIP LRSSWVMTCAYAPSGNYVACGGLDNICSIYNLKTREGNVRVSRELAGHTGYLSCCRFLDDNQ IVTSSGDTTCALWDIETGQQTTTFTGHTGDVMSLSLAPDTRLFVSGACDASAKLWDVREGM CRQTFTGHESDINAICFFPNGNAFATGSDDATCRLFDLRADQELMTYSHDNIICGITSVSFSKSG RLLLAGYDDFNCNVWDALKADRAGVLAGHDNRVSCLGVTDDGMAVATGSWDSFLKIWN Mouse Gβ P62874 GBB1_MOUSEMSELDQLRQEAEQLKNQIRDARKACADATLS (65) QITNNIDPVGRIQMRTRRTLRGHLAKIYAMHWGTDSRLLVSASQDGKLIIWDSYTTNKVHAIP LRSSWVMTCAYAPSGNYVACGGLDNICSIYNLKTREGNVRVSRELAGHTGYLSCCRFLDDNQ IVTSSGDTTCALWDIETGQQTTTFTGHTGDVMSLSLAPDTRLFVSGACDASAKLWDVREGM CRQTFTGHESDINAICFFPNGNAFATGSDDATCRLFDLRADQELMTYSHDNIICGITSVSFSKSG RLLLAGYDDFNCNVWDALKADRAGVLAGHDNRVSCLGVTDDGMAVATGSWDSFLKIWN Bovine Gγ P63212 GBG2_BOVINMASNNTASIAQARKLVEQLKMEANIDRIKVS (66) KAAADLMAYCEAHAKEDPLLTPVPASENPFREKKFFCAIL Mouse Gγ P63213 GBG2_MOUSE MASNNTASIAQARKLVEQLKMEANIDRIKVS(67) KAAADLMAYCEAHAKEDPLLTPVPASENPFR EKKFFCAIL Human Gγ P59768GBG2_HUMAN MASNNTASIAQARKLVEQLKMEANIDRIKVS (68)KAAADLMAYCEAHAKEDPLLTPVPASENP FREKKFFCAIL

TABLE 6 Potential intermolecular interactions within the R:G interface

TABLE 7 Data collection and refinement statistics Data collection*Number of crystals 20  Space group P 2₁ Cell dimensions a, b, c (Å)119.3, 64.6, 131.2 α, β, γ (°) 90.0, 91.7, 90.0 Resolution (Å)   41-3.2(3.26-3.20) R_(merge) (%) 15.6 (55.3) <I>/<σI> 10.8 (1.8)  Completeness(%) 91.2 (53.9) Redundancy 6.5 (5.0) Refinement Resolution (Å) 41-3.2No. reflections 31075 (1557 in test set) R_(work)/R_(free) (%) 22.5/27.7No. atoms 10277    No. protein residues 1318   Anisotropic B tensor B₁₁= −7.0/B₂₂ = 4.7/B₃₃ = 2.3/B₁₃ = 2.1 Unmodelled sequences* β₂ adrenergicreceptor 29^(b), 176-178, 240-264, 342-365 G_(s) α, ras domain 1-8,60-88, 203-204, 256-262 G_(s) γ 1-4, 63-68 T4 lysozyme 161^(c)  AverageB-factors (Å²) β₂ adrenergic receptor 133.5  G_(s) α, ras domain 82.8G_(s) α, helical domain 123.0  G_(s) β 64.2 G_(s) γ 85.2 Nanobody 3560.7 T4 lysozyme 113.7  R.m.s. deviation from ideality Bond length (Å)  0.007 Bond angles (°)  0.72 Ramachandran statistics^(d) Favoredregions (%) 95.8 Allowed regions (%)  4.2 Outliers (%) 0  *Highest shellstatistics are in parentheses. ^(a)These regions were omitted from themodel due to poorly resolved electron density. Unmodeled purificationtags are not included in these residue ranges. ^(b)Residues 1-28 of β2ARwere omitted from the construct and T4L was fused to the amino terminusof transmembrane helix 1 to facilitate crystallization. ^(c)Residue ofT4L was omitted from the construct. ^(d)As defined by MolProbity.

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The invention claimed is:
 1. A nanobody comprising: four framework regions (FR1 to FR4) and three complementary determining regions (CDR1 to CDR3), according to Formula (I) FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4  (I) wherein the nanobody specifically binds to a complex comprising a GPCR and a G protein; wherein the nanobody specifically binds to the G protein and not to the GPCR; and wherein CDR1 is SEQ ID NO: 13, CDR2 is SEQ ID NO: 25, and CDR3 is SEQ ID NO: 37; or CDR1 is SEQ ID NO: 14, CDR2 is SEQ ID NO: 26, and CDR3 is SEQ ID NO: 38; or CDR1 is SEQ ID NO: 15, CDR2 is SEQ ID NO: 27, and CDR3 is SEQ ID NO: 39; or CDR1 is SEQ ID NO: 16, CDR2 is SEQ ID NO: 28, and CDR3 is SEQ ID NO: 40; or CDR1 is SEQ ID NO: 17, CDR2 is SEQ ID NO: 29, and CDR3 is SEQ ID NO: 41; or CDR1 is SEQ ID NO: 18, CDR2 is SEQ ID NO: 30, and CDR3 is SEQ ID NO:
 42. 2. The nanobody of claim 1, wherein the nanobody binds with higher affinity to the complex as compared to binding with the heterotrimeric G protein alone.
 3. The nanobody of claim 1, wherein the complex further comprises a receptor ligand.
 4. The nanobody of claim 3, wherein the complex consists of a GPCR, a G protein, and a receptor ligand.
 5. The nanobody of claim 3, wherein the receptor ligand is an agonist.
 6. The nanobody of claim 1, wherein the G protein is in a nucleotide free form.
 7. The nanobody of claim 1, wherein the nanobody specifically binds a conformational epitope at the interface between the alpha and beta subunit of the G protein.
 8. The nanobody of claim 1, wherein the nanobody a) prevents or inhibits the dissociation of the complex in the presence of nucleotides, or b) prevents or inhibits the binding of nucleotides to the G protein, or c) displaces nucleotides from the G protein.
 9. The nanobody of claim 6, wherein the nucleotides are guanine nucleotides or analogs thereof.
 10. The nanobody of claim 1, wherein the GPCR is in an active conformation.
 11. The nanobody of claim 1, wherein the G protein is Gs.
 12. The nanobody of claim 1, wherein the GPCR is a Gs coupled receptor.
 13. The nanobody of claim 1, wherein the GPCR is a human protein.
 14. The nanobody of claim 1, wherein the nanobody comprises SEQ ID NO: 1, or SEQ ID NO: 2, or SEQ ID NO: 3, or SEQ ID NO: 4, or SEQ ID NO: 5, or SEQ ID NO:
 6. 15. The nanobody of claim 1, wherein the nanobody is comprised in a polypeptide.
 16. The nanobody of claim 1, wherein the nanobody is immobilized on a solid support.
 17. A complex comprising the nanobody of claim
 1. 18. The complex of claim 17, wherein the complex further comprises a GPCR, and a G protein.
 19. The complex of claim 17, wherein the complex is crystalline. 